I would not put firemouths with this guy even in a 300 gallon, they are mild mannered fish with very easily dislocated jaws. I would say go for convicts if you want to try some smaller cichlids in with this beast. Other than that, I'd say for tankmates to avoid anything that looks similar to this guy, such as a full-breed texas or mayan. Large Flowerhorns/Amphilophus will be your best bet. Maybe big pike cichlids if you can find them, as they look nothing like him and they might not be seen as a threat. I would keep all the other fish in the tank first and let them get established, then put this guy in last, so he does not see it as his territory that others are intruding on. Be advised that even in a 300 gallon, this guy might be a killer.
You might have better luck just keeping large non-cichlids with him. Polypterus, catfish, that sort of thing.