I live in the area convicts come from, they will do fine in 75 degrees. They are (as already stated)from from Central America, here in Panama they are quite common (in areas such as Bocas del Toro), where there are many variations, (and species of convict types). Also in Costa Rica and Nicaragua, where they inhabit slow, to fast moving waters, some as cool as low 70s' in the rainy season. I would suggest dither fish as tank mates, instead of other cichlids. Here there are many live bearers that share their habitat, but spend more time near the surface, and fit the bill in your size tank. This would add movement and interest throughout the entire tank, and these fish would not (hopefully) be thought of as competitors (like other cichlids would). Convicts don't generally like other competitors, and since you have put them in first, will see any new cichlid additions as interlopers, and probably try to kill them. Convicts albeit slowly, can grow fairly large, compared to the space you have, and a pair could (with proper care) eventually max out your 55 gal tank.