You could probably have a pair of Rams in a 20 gallon. Ohter cichlids you could have are some dwarf cichlids like Agassiz's Dwarf Cichlids(Apistogramma agassizzi) or Cockatoo Dwarf Cichlids(Apistogramma cacatuoides).
A pair of Julidochromis ornatus (although it is hard to distinguish a male from a female) or J. transcriptus (maybe, need some correction if applicable)