You can probably stock quite a few different species in a non-planted tank. If you have wood and rocks to create site-breaks and delineate territories, you can probably re-create a naturalistic environment as well - not all parts of every river in SA/CA is chock full of plants!
I think the more limiting factor is the size of the tank - there are plenty of SA/CA cichlids that are notorious for tearing up plants and/or eating them - but many of them get too large for a 55g tank.
So keeping in mind you may be better off with smaller growing species, I think you could choose from the following (and more)!
- Keyhole cichlid
- Bolivian Ram
- Severum (some species may get too large).
- Blue Acara / Electric Blue Acara
- Redhead Tapajos
Keep in mind that many of these fish, especially the CA's, are highly aggressive, so single specimens may be necessary (a wet pet situation).
I also know that some of these fish often pop-up in planted tanks (ANgelfish), but that doesn't mean you have to keep them in planted tanks. Many breeders maintain bare bottom tanks with a breeding cone. It all depends on what you want!