Apistos would be a good option in a 20. Also several africans from Tanganyikia, several of the lamprologines would also be good, like the brichardi, lelupi ect. The smaller julies like dickfeldi, transcriptus, or ornatus.
Any idea of what you would like to see in your tank? Its also a good idea to scope out the LFS to see what you can easily get. If you can get it localy it will be much cheaper.