They do get very big, and are very aggressive. Fish is their main diet - but crocodilians enjoy snacking on turtles, snakes, frogs, basically whatever they can catch.
If the caiman is well fed, it might co-exist with larger fish - but in situations where the fish can easily actually swim away - ie: in one of those giant exhibit aquariums.
You'd be much better off choosing something that is practical - Water Dragons would be sweet as an alternative.