In a 55 gallon you can put together a very nice Tanganyikan Community.
Decorate with rocks on one side of the tank, and cluster the shells on the other side, leaving swimming space in the middle. Sand is the best substrate to go with, I like beach sand sold as play sand, it's clean, cheap, and looks good.
For the rock dwellers you have a nice assortment of Neolamprolines, Altolamprolines, and Julidochromines, as well as a couple of others. Depending can keep two pairs of rock dwellers. Most of the Tang rock dwellers are monogomous. Purchasing 4 juveniles and allowing a pair to form naturaly is the best way to achieve a solidly bonded pair.
On the other end you can keep a couple pairs of shelldweller like Brevis, a harem of 6 or so of something like Ocellatus, or a small colony of Multifasciatus numbering around 7 or 8.