awesome thats plenty of room. You would probably be able to add just about any of the small/medium cichlids(cons, salvini, jack dempsey, etc.). As long as there isnt a big size difference when you add them. I mean if your firemouths are only like 1"-3" you probably wouldnt want to add anything too much bigger or the fm's would most likely get beaten up. Or if your looking for dithers other than cichlids, silver dollars work nicely. It all depends on your preference. If you had something in mind, post what it is and im sure someone can help you further.