The two actually do fight often, though Luka doesn't usually start the fight unless it's real bad. He'll defend himself and Xiao knows this well, he's just generally a better fighter and not held back by 2 tons of tit.

Between feline folk the drawing of claws is a very "serious" thing, you'll see it in life or death situations when adult felines fight. Claws are treated a lot closer to knives by the law in these cases as well. Between the kitties it's a lot less common, you'll usually see punches or palm strikes.

I could definitely see a situation where Luka instinctively draws his claws on Xiao and takes an actual swipe, it'd surely come as a surprise for the Leopard, and it'd be a very painful thing for his "friend" to ever do such a thing to him. Though, Luka's claws are currently dull so it'd probably end up as a pretty humiliating moment.