Really depends on how big your tank is.... and what you might have your heart set on. You probably want to have at least a 55 gallon before considering another larger cichlid. A firemouth should play nice with most of these guys.
While not geographically correct, I *think* tiger barbs might do OK if there are enough of them in there. Tiger barbs can be kind of nippy towards other fish (and mean to each other) unless you have about 6 or more. I'm considering putting our tiger barbs (as dithers) in with a severum in our "hypothetical new tank"... need to research that a little. We have 10 of them in our 29 gallon tank. I do wonder if they have good "dither" qualities, though. Ours are a little skittish and like to play hide and seek. Very active and fun to watch, though.
Our red-eyed tetras are great dithers. They just hang out in the top level of the water column, not a care in the world. I sorta fear they may be cichlid food when our severums get a little bigger.
-Ryan