to work on their personal relationship, and they both selfishly try to gain the upper hand in their marriage.

Curley is an insecure, combative person who likes to pick a fight. When he first meets Lennie and George, Curley says, "Let the big guy talk" (when George continues to speak for Lennie). Curley seems to be itching for a fight, and he dislikes Lennie because Lennie is a big guy. Curley, a small man, feels inferior because of his small stature. He wears a glove and coats his hand with vaseline to keep it soft for his wife. Curley clearly worries that his wife might not find him attractive. 

One of the ranch hands says of Curley's wife, "she got the eye." Even though she was just married to Curley, she is flirtatious with other men on the ranch. She is also constantly stopping by the bunkhouse, ostensibly looking for Curley but really looking for attention. She enjoys attention, and she later shows up at the bunkhouse wearing red mules topped with ostrich feathers. She is heavily made up and paints her nails red, and she clearly likes to receive attention from men and is lonely as the only woman on the ranch. 

Curley and his wife have a very turbulent relationship in Of Mice and Men.

Curley is possessive regarding his wife.  He shows this in chapter 2, when he is repeatedly asking if anyone knows where his wife is.  He suspects many of the ranch hands when it comes to his wife, including Slim.  When talking to George, Whit suggests there might be a sense of turbulence between both husband and wife.  He speaks about how Curley is quite anxious about his wife, saying that Curley has "yella- jackets in his drawers."  At the same time, we can presume that Curley does not treat his wife well when he is with her.  When she is talking to Lennie, she tells him that Curley's "not a nice fella."  The way she says it reflects personal knowledge about what he is capable of doing.  It is evident there is not much in way of happiness between them.

Steinbeck does not depict the couple in anything resembling a healthy relationship.  Whit perceptively says that both of them are akin to two ships passing in the night:  "He spends half his time lookin’ for her, and the rest of the time she’s lookin’ for him.”  Both of them are not shown as being emotionally settled with one another.  This reflects how Curley and his wife do not find much in way of happiness in their relationship.

In Steinbeck's novella Of Mice and Men, both Crooks and Curley's wife are marginalized.

Much of the time, Crooks is alluded to simply as the "stable buck" in the narrative. Candy initially speaks about him in an objectified manner as he tells George how the boss has reacted to their not having reported to work in the morning:

"He was sure burned when you wasn't here this morning. Come right in when we was eatin' breakfast and says, 'Where the hell's them new men?' An' he give the stable buck hell, too."

When George asks Candy about his last sentence, Candy explains that the "buck's a n****r. . .The boss gives him hell when he's mad." So, Crooks is a verbal whipping boy.

Additionally, he is isolated from the other ranch hands as he is made to live in the barn, apart from the others in the bunkhouse. So, he occupies himself with reading when he is not working because he is also not allowed to play horseshoes or cards with the other men. When the men go into town to drink and engage in other activities, Crooks never goes then, either.

Curley's wife, too, is prevented from interacting with others on the ranch. Just like Crooks, she is objectified; she has no other name but that of being the genitive of Curley, her husband. When she first appears in the narrative, she stands in the doorway of the bunkhouse in a seductive pose, the only position which gives her any attention. She can only use her womanhood to gain any attention, and so she becomes an Eve to tempt the men and manipulate them. However, she duplicates Curley's mistake of failing to comprehend how uncontrollable is the fear and irrationality of Lennie. 

Both Crooks and Curley's wife play unique roles in the society of Steinbeck's novella. Crooks embodies the isolation, loneliness, and marginalization of many during the Great Depression while Curley's wife is a vehicle by which Steinbeck contrasts the way that various men act. Moreover, she acts as a disruptive force in the fraternity of men.

Curley's wife and Crooks share one important characteristic - powerlessness. Within the social hierarchy of the ranch, these two occupy the lowest levels of status, importance and freedom. 

 ...the theme of loneliness is further explored in the solitude borne by Crooks and Curley's wife... Both these characters crave company and, as Curley's wife says, "someone to talk to."

Curley's wife is expected to stay in the house all day. When she fraternizes with the men on the ranch, she is rebuked and becomes the target of both fear and derision. The men cannot trust her and say that she is a "tart", which may be true, but she is also isolated and lonely and sees no positive way to achieve a social life on the ranch. 

She once dreamed of being an important person, a movie star. The notion that she could have been a star remains important to her despite the fact that the man who said she could have been a star was lying to her and, ultimately, she knows that he was lying. 

Crooks is also isolated and expected to stay in his place which is well-defined both socially and physically. As a crippled person, Crooks has little literal power and this mirrors his social position. 

Differences:

Obviously, she is female, he is male. He is black, she is white. Although these are simplistic, they are worth noting because these are dramatic differences for the 1930s era. Today, men and women cross gender lines in terms of stereotypical roles and expectations. The same is true culturally for blacks and whites. Often, stereotypical assumptions can no longer be assumed because blacks and whites has assimilated into each others' previously singular cultural norms.

Curley's wife is put on a sort of spotlight and given value because she is the woman of Curley's affection and worth. She is given the attention of the men because they dare not cross paths with her for fear that they will be accused of flirting with her. She is treaated like an object to be possessed.

Crooks on the other hand has worth and value for other reasons and at least has a name. Crooks is identified by his role on the farm. For him, we see his permanent status in the items he has gathered over time. He has his own room which could further give him importance, but he views it as a way he is separated from the others.

Similarities:

Both Crooks and Curley's wife are intended to be permanent residents on the ranch. This is different than many of the other characters who travel from ranch to ranch for work. Both are outside the majority of the characters for one reason or another. Both are extremely lonely because of their differences from the rest of the group. They both long for friendship, but alas, no one will have them. They both get the attention of Lennie, a central character who demonstrates more humanity than the rest of the characters. This attention serves to fulfill their longings momentarily.

Crooks and Curley's wife are treated as outcasts, and thus experience terrible loneliness in John Steinbeck's novella Of Mice and Men. Crooks is the stable buck on the ranch. He is a black man on an all-white ranch in the 1930s, so he is the victim of both segregation and racism. Curley's wife is the only woman on a ranch full of men. She is very young and pretty and quite out of place.

Crooks is first described in chapter two by the old swamper, Candy. Because he's black, Crooks has his own room in the barn. Steinbeck uses the words "proud" and "aloof" to portray Crooks. He may be aloof because he senses the prejudice of the some of the men. He isn't often allowed in the bunkhouse where the white workers live. Candy relates the story of the time Crooks got to come into the bunkhouse on a special occasion. Crooks ended up fighting one of the workers, presumably over a race issue. Candy describes the fight:

"They let the nigger come in that night. Little skinner name of Smitty took after the nigger. Done pretty good, too. The guys wouldn't let him use his feet, so the nigger got him. If he coulda used his feet, Smitty says he woulda killed the nigger. The guys said on account of the nigger's got a crooked back, Smitty can't use his feet.

I had to cut down the original question text, as there were multiple questions present.  They are all real interesting and I encourage you to repost them separately.  In my mind, I think that Lennie and Curley's wife being along at the start of Chapter 5 is significant because it brings together two fairly sad creatures in one instant.  Lennie's hopes of a life where he can "tend the rabbits" and be surrounding with creature that allow him to enjoy a state of being in the world are set against Curley's wife's dream of being in "pitchers" and being someone of importance and significance.  In this particular instant, the striking similarity of their dreams and their own potential for loneliness because of their dreams' denial is brought out in full force.  At the same time, the differences between them is also present in that Lennie does not possess bitterness about the deferral of his dreams.  Perhaps, this is because he lacks the capacity for it.  Yet, Curley's wife is bitter and she is distraught that her own dreams were not recognized.  I think that another significant difference that is evoked in chapter 5 is how Curley's wife lives her life with the consciousness of her dreams being negated, while Lennie possesses the childhood innocence that at some point, in some way, his dreams can be fulfilled.  The moment where both of them interact through touch is one where neither one understands the vulnerability of the other, and where Steinbeck might be asserting that the cost of the denial of our dreams could be our ability to understand another person's own pain when they experience what we have.

In the novel Of Mice and Men, Curley's wife does not relate well to the themes in the novel. She does not desire the lonely life she lives. She does not enjoy the isolation. She has always desired the more exciting life. She dreams of acting. She dwells on the past when she was observed by some who thought she could make it acting:

But she is pathetically lonely and once had dreams of being a movie star.

The ranch hands are gone all day working. They play games at night. Curley's wife feels left out. She obviously desires companionship. She has no interaction with society.

She lives out on the ranch, away from women her age. She is all alone on the ranch. She feels alienated from society. She has no one to confide in. She is basically without a friend.

For this reason, she reaches out to the field hands. She has no choice but to talk with Lennie. She uses him to get the attention she is so desperate for. Because she played with Lennie's emotions, she winds up with a broken neck. It was an accident.

Crooks and Curley's wife occupy very different social stations in the novel. Crooks is the African-American stable hand who lives on the lowest social rung at the ranch. He has the least power and respect, even among the hired ranch hands. He is old and physically broken. Curley's wife, in contrast, is white, young, and beautiful. As Curley's wife, she represents the ownership interests of the ranch. She does not have to work.

Crooks and Curley's wife, however, have some significant similarities. Both of them are isolated and lonely. Because of racial prejudices, Crooks is not allowed to live in the bunkhouse with the other hands and is not allowed to come out of his quarters in the harness room to join them except at Christmas. Curley's wife has no one on the ranch, except her cruel and controlling husband, to keep her company. She lives her days alone, cut off from human contact, with nothing to do. Because she is a woman, she is out-of-place in the man's world of the ranch, and because of her husband's jealousy, she is supposed to stay away from the men altogether.

Two characters isolated by their singleness, Curley's wife and Crooks are on the fringes of the male society of Of Mice and Men. For them, there is no fraternity, no sharing of work or enjoyments or dreams. While Curley's wife seeks attention by flashing her red fingernails and arching and twisting her body before the men, Crooks retreats into his books from the attention he knows he will never receive, for his barrier of race is greater than any barrier that keeps Candy's wife from socialization. The depth of this barrier is exemplfied in the scene in which he tries to prohibit Curley's wife from entering his room.  For, she turns to him scornfully,

"You know what I can do to you if you open your trap?"

and Crooks retreats upon his bunk "and drew into himself." After Curley's wife leaves, Crooks tells the men that perhaps the should leave as he does not want them in his room any more.  He tells Candy to "forget his idea of working there":

..."Well, jus' forget it.   I didn't mean it. Jus' foolin'. I wouldn'want to go on placec like it.

Forlornly Crooks sits on his bench and looks at the door for a moment.  Then, he defeatedly reaches for the liniment bottle with the knowledge that he will always be part of a race that is apart from the others.