The story is about two itinerant farm laborers who have a dream of owning their own home with a couple of acres where they could raise their own food and be independent. It could not have been about a male and female bindlestiff because there were no female itinerant farm laborers at that time. Steinbeck must have realized that he had a plot problem. How would it look for two men to be dreaming of sharing a home together? It would look as if they probably had a homosexual relationship. The normal scenario, not only in Steinbeck's time but going all the way back to ancient Egypt and Babylonia, was for a man and a woman to own a farm together and to have children who would work on the farm and take care of them in their old age. A good fairly modern example of this is Norwegian Noble prize-winner Knut Hamsun's great novel Growth of the Soil.
Steinbeck came up with the idea of making one of the two main characters mentally retarded. This would help to explain why George was willing to share a little subsistence farm with him. Steinbeck made Lennie exceptionally strong in order to show that he would not be a total liability but could do more than his share of the heavy work. An important collateral advantage of making Lennie feeble-minded was that George would have to explain everything to him and sometimes explain it all over again; while in the meantime he would be explaining everything to the reader and the audience.
According to the enotes Introduction in the Study Guide, Steinbeck intended to turn his story into a stage play. It can be seen that the novella reads almost like a "treatment." It is extremely short. I has only a few very simple sets--a bunkhouse, a barn, and Crooks' room. Most importantly, there is a heavy reliance on dialogue, which would make it easy to convert the novella to a play. There is hardly any prose exposition; the exposition is conveyed in the form of dialogue. As good example is the story George tells his retarded friend in the opening chapter.
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake and then they go into town and blow their stake, and the first thing you know they're poundin' their tale on some other ranch. They ain't got nothing to look ahead to."
I feel sympathy for these bindlestiffs, but I can't help feeling that their dream is fatally flawed. If they did manage to buy that farm they would be able to raise their own food, but they would have nothing but food and shelter. It would not only be a boring existence, but they wouldn't have a penny to buy anything they couldn't raise themselves. They would need shoes and clothing. They would need seeds and fertilizer. They couldn't sell any of the vegetables or animals they raised, or any of their eggs or milk, because everybody else in the region would have the same things in abundance.
The only way they could get their hands on some cash money would be to work for other people. Most likely they would pick fruit during the summer months--which would mean that they were only semi-independent and would have to work harder than ever, because they would have to tend to their own farm while they were working for others.
The story doesn't really illustrate Steinbeck's thesis that the best laid plans of mice and men get twarted. By turning one of his two characters into a potentially homicidal retard, he is setting it up for the plan to fail.