Cat sharks are relatively docile, and you can keep most fish with them. They are not active hunters, and will bypass a fish for the most part. They enjoy eating molluscs and crustaceans for the most part. It's more a question of which fish do you want to keep away from cat sharks, such as triggers and eels, and some of the angels, as they all will nip at the shark and bother it, or in the case of the triggers and eels, possibly eat it. You're pretty safe, though, in not putting anything in there that it can fit in it's mouth. A quick anecdote: the first time I hatched a banded shark (a member of the cat shark family), it was in my reef, and I had these little blennies that looked a lot like tiny lawnmower blennies, except they were not aggressive towards each other; I would feed my shark tiny pieces of shrimp on a skewer, and the little blennies (less than 1/2" each) would steal it out of the sharks mouth.