Single combat is for when there are two combatants or two sides - one character against another, one gang against another, or, sure, a PC vs a gang - but nobody is fighting for anything except the fight. If either side has an objective other than to straight-up murderize the other side, use the seize & hold moves instead. Single combat is for formal, scheduled fights on neutral ground, like duels, bouts, cage fights, and Gangs of New York.
The chaotic free-for-all is for when there aren't sides, just everyone out for themselves, or for when there are so many sides that they break down completely as soon as bullets start flying. It's for when a mob turns its violence inward instead of outward.
The general rule is, unless you're absolutely certain it's single combat or a chaotic free-for-all, you should probably use the seize & hold moves instead.
-Vincent