In most gang fights, there were no rules. Knives, chains, bottles, pipes, pool sticks and "sometimes even heaters" were used, especially by Tim Shepherd's gang. But in this particular rumble with the Socs, there would be no weapons, only "skin" (fists). This was confirmed by Cherry when Ponyboy went to confer with her.
"They play your way. No weapons, fair deal. Your rules."
Each side chose a man to start the fight and throw the first punches. Because Darry was the biggest and strongest, he volunteered for the greasers.
"I'll take on anyone."
An old high school friend, Paul Holden, agreed to represent the Socs. The two stalked each other for a few moments, circling until Holden threw a hard right hand "that would have felled anyone by Darry." Weapons actually were used--one of the Brumly boys used a pipe, and the leader of that gang was "working over one of his own men" for doing so.
The fair rules were no weapons, only skins/bare hand making it fair.