20 gallon long... what would be a good fit?

#1
Hey everyone, I know this question has been asked before but I kinda wanted to ask a few more specific questions. Anyway, I want shellies for my 20 gallon long. It will have a fine sand substrate and slate. How many shellies could I put in this sized tank (and what species) and also could I add another pair or trio of some other cichlid to swim around further toward the surface. Particullarly, are there any yellow cichlids that would be suitable for this sized tank? I know the tank is small but it has to fit on a countertop. Thanks for the help and sorry I don't know more about African cichlids, I just decided to take the plunge about a week ago. I appreciate any input.

BTW, my local fish store is loaded so I can find pretty much whatever you recommend. Thanks again.