Do you know if your firemouths are male and female? Pairs of any cichlid typical increases the aggression factor. Whenever suggesting cichlid tankmates you'll often find conflicting opinions based on the individual results of others. So if you move/get rid of the other cichlids (angel, ram, brichardi) you should look for other medium sized cichlids. Convicts are very common, the pink form would contrast well with the others.
or perhaps another large single, green terrors being popular. Of course with large cichlids you run the risk of getting a truely obnoxious one that turns your tank upside down and destroys everyone else.
The geophagus genus is gaining in popularity. They're generally peaceful but keep in mind geophagus translates to earth eater. They will move a lot of gravel, rearranging the tank as they please. But that's not unusual for cichlids.
The options are many, availability or your persistance to find a species may be the limiting factor. Get an idea of a few that appeal to you and then see what's available. Don't forget about those old things called books

I find them easier when trying to view a broad spectrum of fish rather than searching one species at a time. Also note that when researching SA cichlids the genus names have been shuffled pretty recently so you will likely find synonyms for the same fish. Next you will need to trust your retailer because many species look very similar, when they're washed out juvenilles in a dealer's tank.