I wouldn't add just one more.They are less likely to be aggressive towards each other if you get a few.It's best to get a group of youngsters and let them grow up together. if you end up with to many males, you can take a couple back to the store.
I have a tank roughly the size of yours with 11 thorichthys helleri(from the same family as firemouths just in case you don't know)
I have 3 males in there and the rest are females.
The tank has been set up for nearly two years and I have no problem with aggressive behaviour.
I also have a group of swordtails for the upper levels of the tank.
You could do the same with firemouths in your tank.