What kinds of rules do gangs members follow in fights?
Two-Bit explains in The Outsiders that "a fair fight isn't rough" (page numbers vary by edition). He defines blades, chains, heaters, pool sticks, and rumbles as rough, so Greasers avoid these things. Greasers do not use weapons, but they allow skin fighting. As Two-Bit explains, "There's nothing wrong with throwing a few punches." Greasers try to stick together, but if they fight among themselves, it's always one-on-one. Socs, on the other hand, will gang up on Greasers or have rumbles among themselves. For example, Johnny is beaten up when four Socs jump him, and one beats him up with a ring on his hands. At the beginning of the book, Ponyboy is jumped by five Socs in a Corvair, and they pin him down and hold a knife to his throat. Greasers, on the other hand, will not try to gang up on or jump Socs (unless they have to in self-defense), because they believe in fighting fair.
As the previous post noted, the combatants in the rumble in The Outsiders previously agreed to using no weapons--no knives, chains or heaters. It was not particularly a clean fight: Some fought two-against-one, others kicked, pulled hair, and pounded heads against the ground. Some previous rumbles between the two groups apparently included knives and chains, but both the greasers and Socs had decided against the use of weapons in this particular fight--possibly because of Bob's death from Johnny's knife. They may well have figured that enough blood had been shed.