you could put 2-3 more firemouths in there, or even a few blue eyed cichlids or flyer cichlids. you could also try some dwarf puffers ( but be prepared to take the puffers back if they get too out of hand). Are your firemouths agressive? because from what I've read about them, they can be very peaceful, or very agressive.
what type of pleco do you have? if its the ones the stores sold as "Commen pleco" then you may want to take him back, as they can grow over 2' long.