Since there's no heater, what temps are we talking about?
In my 10g I had Elassoma sp. with some pygmy killies (L. ommata). Both are supposed to be somewhat easy to breed given the right conditions, although breeding is less likely when put together for obvious reasons. I didn't have any luck with the pygmy sunfish though...they never liked flake, barely touched frozen, and I was too late with live foods so they eventually withered away after being stressed from a move. Others seem to have better luck though, so it's probably just me...figures.
Anyways, I really do like the pygmy killies though. The males have a striking gold color when mature and in a peaceful setting.