As for theoretical tank mates.... anything from Lake Malwai would work
I'd stick with lake tang species. In a 20g long you could probably keep a pair of shellies and a pair of small julies (J. transcriptus or J. ornatus). the key would be to have a reef at one end (rocks stacked almost to the top of the tank) and open sand with shells at the other end. Shellie species vary on how much territory they require. Brevis only need about 4-5in around thier shells while multis seem to take the entire tank.