In my experience FMs can be very aggressive at times. My firemouth likes to play shy but will easily put the smackdown on all my other cichlids. He would always fight with the red oscar that i had and the O was double the FMs size. The only fish he would backdown to was my salvini who owned the whole tank pretty much. I got another firemouth only to have to rehome it because the two firemouth were battling it out for a few days straight and it was all hell let loose. You can try a pair in the tank but i personally wouldnt do a trio after the experience that ive had with them. Even then just be ready in case one decides to go postal on the other and you should be all set. One of my favorite cichlids though watching them flare makes it all the worth while of keeping them in a tank. So i saytry to get a pair then add some dithers for color and aggression. (6 tiger barbs worked wonders for me) and just have the tank set up with plenty of hiding spots!