I would say you don't have any more room for additional cichlids, but you could try some small schooling fish (danios, tetras, barbs). Keep in mind that you may lose a few as snacks. Are your cichlids m/f pairs? Do they ever spawn? If so, the aggression level might be too high for other fish to be introduced.
Edit: a mod might want to move this over to Central Americans
Edit: a mod might want to move this over to Central Americans