They will grow to be about 8 inches long. It might work but they would probably like a little more room. If you want cichlids you could do rams or angel fish. Angels would love a 30 gallon high.
Could 1 Firemouth and 1 Convict work? If so what is more peacful male or female? I might also try to put an angelfish in, if it looks like it getting picked on Ill move him to my 55 or take him back to the LFS. How do you sex convicts?