there is not much you can put in a 20 gallon as far as bigger cichlids go. have you looked at the shell dweller cichlids? there pretty cool. kribs, rams would work but you couldnt have many.
Firemouth Cichlids need a minimum of 40 gallons. You could do a pair of rams, look for dwarf Cichlids. Apistograma would also work.
I thought you wanted bigger cichlids sorry my bad. I think I spent $7 on my rams from petsmart. thanks cichlidnut.