My tank is 58 gal and I have two in my tank and a blue spotted jawfish, flame angel, two clowns and a royal gramma. They all seem to get along very well. Even the jawfish will come out of his burrow and fight for his share of the food.
When I ordered the firefish I tried to ordered a mated pair. The guy told me that unless they are mated they will fight. I am not sure how true that is. I do see the larger firefish sometime nip at the other one. But nothing serious at all.