A newly matured pair of Australoheros red ceibal would be my choice because they wouldn't need a heater, in fact, fluctuation in temp seems to spur them on.
I had a pair spawn in an outdoor tank, choked with algae, water temps would be high 80sF during the day, 50sF at night.
I never fed the tank, the pair and fry lived off algae, and whatever else fell in.
A 10 gal would probably be to small after the initial breeding the first season, because they grow very quickly.