Answer: The rules of the rumble are simple: no weapons are to be used (just fists), and the first to run loses. But the Greasers do indeed carry weapons such as knives, but they're only to be used for self-defense in the event of an attack. An organized rumble obviously wouldn't fall into that category.
Explanation: How greasers play in a rumble- The greasers prepare for the rumble by showering, putting on clean clothes and oiling their hair so it looks extra greasy. On their way to the vacant lot, they pump themselves up, doing flips and chanting.