Both IC and OOC there are no limits on any kinks in UFL. Ideally, you should be playing it like intended - whoever wins gets all the spoils, and can do anything he or she wishes to the loser of the fight.
By default I would suggest to focus on kinks that actually fit the atmosphere of underground\gladiator fight. Which are violence, hatefucking, public humiliation. I would suggest avoiding kinks on the weirder side. I'm NOT judging anyone here, but including diaper play into fight RP is probably not a very good idea. But, if you wish, sure, go on.
All that said, however, all standard F-List RP rules apply here. So ideally you should probably ask your partner about kinks, as you would in your regular RP. Because at it's core UFL is just a wrapper around regular RP :)
And even though you can't always know which F-List character is behind the Stage Name, you can see their kinks loaded under their fighter card.