A "group" of Firemouths will torment each other constantly (no matter how big the tank is) until you get a mated pair and they (probably) kill the rest of the group. I have 4 and they're constantly chasing each other (The biggest one took half of the tank and the other 3 fight for the other half). They completely ignore other fish but are very annoyed by their own species.
This is weird because in the small LFS tank they were very peaceful and quiet but once I took em home they got territorial. I guess they had nothing to fight for in the empty 10 gal box lol
I started with 2 Firemouths but since the bigger one was making the other miserable I decided to buy 2 more in hope of giving the smaller fish a chance to "blend in", kinda like the method used for African Cichlids... but now the smaller one is chasing the "new" smaller fish and it's basically a never ending cycle.
The good thing about it tho is that you get a lot of gill flaring, constant movement and the dominant male will color up really nicely... you would get none of this by keeping a single Firemouth. They also never seem to really hurt themselves... it's more like the "barking dog that never bites" thing.