What are some of Finny's rules (the ones implied as well as stated) in A Separate Peace?
In Chapter 3 of A Separate Peace, Gene narrates that Finny's life is regulated by "inspiration and anarchy."
Despite this anarchy, Finny does value a set of rules—his own. Gene notes that Finny states his rules as though they are commandments. For instance, Finny decrees,
"Never say you are five feet nine when you are five feet eight and a half. . . . Always say some prayers at night."
In this same chapter, Finny creates the game of blitzball. One of the rules of this game is that no one can use his arms when knocking down the ball carrier. "No elbowing allowed either." He adds that it is fine for the ball to touch the ground when it is being passed. Also, since there are no teams, it is fine to knock down anyone and to escape by reverses, deceptions, and
...acts of sheer mass hypnotism which were so extraordinary that they surprised even him.
Phineas obviously is spontaneous, breaking rules of the dress code and creating games with his arbitrary rules. After he is injured from the fall off the tree limb, Finny attempts to adjust the laws of human nature to align with his concept of friendship relative to Gene.
In chapter 1, Gene notes that Finny always traveled in groups, particularly large ones. This likely implies his popularity, especially coming from Gene's persepective.
In the end of chapter 1, Finny also refuses to wear the school issued pajamas. They weren't military enough. This shows Finny had an affinity for the war and wanted to model or mirror the life of a soldier.
Finny has a ritual about saying prayers at night too.
Another common phenomenon for Finny that takes place throughout the first several chapters is the idea of breaking schools rules and then talking his way out of trouble or possible punishment. He does this in chapter 1 by skipping dinner (an dragging Gene with him), and he does this by wearing a pink shirt to the headmaster's tea in chapter 2.
By chapter 3, Finny creates lots of rules for Blitzball. As Gene notes all of the rules seem to be intended to make Finny look the best.