What about a Flier cichlid? Mine is very friendly. Or any acara, they have a lot of personality for a fish that size. IMO way more than firemouths. I also like Sajica, Spilurum and Convicts though. Or go unheated, save money, and do some kind of Australaheros - they also have a lot of personality, or even if you're not stuck on cichlids, do a sunfish. They have a lot of personality and a lone one should be fine in that size tank.