If you want to keep small South American (not Central American) cichlids, I don't think you could find a better bottom feeder than Corydoras catfish. Come in a big variety of colours sizes and patterns.
SA cichlids in a 55G I'd go sand, driftwood, plants, and any combination of Bolivian rams, blue rams, keyhole cichlids, gold dwarfs, or any of the innumerable species of Apistogramma. Could try angelfish or Mesonauta too. Small SAs would be the best option if you want a nicely planted display.
For something slightly bigger you could get a group of smaller eartheaters like Geophagus pindare or 'Tapajos' orange head.
For small Central American cichlids in a 55G I'd do a similar setup of sand, driftwood, but only the hardiest plants, and maybe two pairs from any of the following: convicts or relatives, firemouths or other Thorichthys, blue acaras, rainbow cichlids, sajica.