There are a couple smaller carnivores around, you could keep 3-4 whimple's piranha,3 red belly piranha, or 3-5 hollandi piranha, don't expect a horror movie level of agression but they will eventually kill anything you put in with them. If you want some local natives you could keep a pair of shellcrackers or bluegill and a decent sized bullhead, they will happily munch minnows and you won't need a heater. If you don't mind mix origins you could keep a dwarf pike cichlid, 3-5 blackfinned celebes half beaks. a couple of the smaller leaf fish. and a couple of cory cat or a small species of pleco cat, and even 3 redhook myleus. The halhbeaks, dwarf pike cichlid, and leaf fish will all take small fish and you wouldn't have a single species tank. good luck.