A 45 gallon tank is likely too small to house a pair of mature firemouths (I assume you mean male/female) with another cichlid. If you add a fish that can handle the aggression of your firemouth pair (there are lots of aggressive cichlids that could do that), it will likely dominate the tank at the expense of the firemouths. If you get something too peaceful, the opposite will occur. Adequate space is one of the main factors influencing the aggressive tendencies of cichlids. A 45 gallon tank is OK for your firemouth pair, but I doubt you will have long term success if you add another cichlid. It might work for a while if your fish are young. You could try something like a convict cichlid, they can handle themselves. But be prepared to move the addition out if the aggression builds. Better to have a happy pair of firemouths than a combat zone with a bunch of tattered and stressed fish. Have you considered dither fish instead of a cichlid? For example, a group of giant danios or similar?
