I was going to say that. The good thing about rainbow wolf fish is that they stay small, and are only aggressive towards other wolf fish, so they can be kept with tankmates as long as they're too big too eat.
You could also have some bichirs in there (as long as they're the upper-jaw species which only get to about 12" or so). You can choose from senegals, ornates, etc.
You could also fill it full of tiny fish. Maybe even do a planted aquarium (it's not as hard as it sounds, though not as easy as some people on this site make it seem).