Speaking of the people who were fighting, they were just controlling the ones getting beaten up, claiming they were breaking up the fight, which isn't illegal even if done selectively.
So, the real assaulter was only one person. Moreover, he didn't cause any serious injuries, so even if the victims were relentless in seeking detention, he wouldn't be detained for many days.
Of course, the few ruffians probably wouldn't dare to be relentless, because if they made too much fuss, they could expect to be permanent guests in the hospital, beaten every time they were discharged, something these people were certainly capable of.
However, such things also depend on the person. If, at this moment, these thugs weren't thugs but ordinary people, the police wouldn't let Cuntou and the others off the hook.