a twenty gallon tank severely limits both the kind of cichlids you can have and the number.
to begin with, a pair of convicts will fill a twenty gallon tank .. that means NO tankmates.
next, don't mix african cichlids (electric yellow - Labidochromis caeruleus) with central american species (convicts).
a twenty gallon tank is too small for almost ALL African cichlids save for the "shell dwellers" such as Neolamprologus brevis.
don't buy ANY cichlid you don't know the name of and have thoroughly researched prior to purchase. a "white cichlid" that you don't know the name of could turn out to be a 16 inch monster that you'll kill in short order in such a small tank.
test your tap water for gh, kh and pH and consider cichlid species that prefer your water chemistry. for example, if your tap water is hard and alkaline, the african shell dwellers are a good choice. on the other hand, if your water is soft and relatively soft (mineral free ... low TDS), than Apistogramma would be an ideal cichlid for you.
other reasonable options for a twenty gallon tank: Pelvicachromis pulcher, Taenicara candidi, Tateurndina ocellicauda