1. Temperment. A firemouth and a con could get along in this tank, ive kept firemouths and cons together for a while, but eventually the cons killed all my firemouths. But that was in a 55g with lots of each fish, but from the stand point of temperment and aggressiveness they can get along, but cons are more aggressive so the problem you might have will be with the con attacking the other fish. Personally i would go with a lone convict or firemouth with a few rasboras, or other like fish. You could also maybe go with a bredding pair of either fish.
2. Planted tank. Convicts are notoriously known as aggressive diggers and can change a tanks decor in a matter of hours, if they feel like it. I have pea sized gravel in 1 of my 55g about 1.5" from bottom, and they dig all the way down to the bottom glass the next day after i rearrange the tank. IF you have Planted plants in there they will uproot your plants in no time, this does not necessarily apply to all convicts but most, and will probably be a problem.
Im not sayin do one thing or another, im just giving you my advice from personal experience, and let you decide on what you'd like to do, and if you'd like to risk gettin plants uprooted and maybe losing a few fish, be my guest.
