Don't get a common pleco they will grow too large for that size tank. Maybe a bristlenose or a pleco that doens't grow large. You could maybe get away with one medium sized central or south american cichlid (4-6 inches) but as the convicts grow and spawn (which they will if you have a male and female) they will become more billigerent. They will rule that tank. Personally I would only keep the convicts but if you get another fish make sure there are adequate hiding places and the new fish has the same tenacity as the convicts. I have kept convicts and I have seen them dominate fish twice their size.