I've kept them together before and no problems. I suggest having a couple of hiding places for the firemouths.
The severums get much larger than the firemouths and can get very mean and nasty. I've always read and been told that severums are gentle giants but all six that I have owned have been big nasties.
My firemouths bred and ended up owning half of my 55g tank even though they were the smallest fish in the tank.
Never has any injuries though. A bit of fin nippage but that's expected.