(I replied to this in the General Freshwater section too.)
Some people will have issues mixing an african and a central american cichlid in the same tank on account of them requiring different water parameters (pH, hardness). I once had a firemouth and a yellow lab in the same tank for a few years and both fish did great. In fact, whenever I added anyone new to their tank, they both bullied the new guy while they ignored each other. I would worry about the difference in size though. Both of my fish were the same size when I put them together. A 3in. yellow lab will be a handful for your 1.5in firemouth unless it's just really full of p*ss and vinegar. A 55gal is plenty of space for 4.5in of fish to get away from each other though. I would try it, but keep them monitored for aggression issues.
Edit: That wasn't a yellow lab, but an orange zebra. My bad, but I would still try it.