Go with Bolivian Rams. They're cheap at about $5 each, breed easily, accept a fairly wide range of water parameters, and have tons of personality. I have 4 adult in my 55g; three females and one male. They get along well except for when the pair is spawning. While they're spawning, especially when they have wigglers and free swimming fry, the mother will be very aggressive towards the other two females. She doesn't seem to mind the other fish in the tank, just the other rams. For that size of a tank, I would recommend trying to get a pair or buying a few and waiting until a pair forms and then selling or returning the others to your lfs. That way you can have them spawn but not have to deal with the aggression that comes as a result of it.