I think so. I have always said a 55 is the minimum for a breeding pair of convicts if they are to live their full lives. I have yet to read or hear about anyone keeping a pair alive to full growth in a twenty. They are just down right nasty for their size and the firemouths are not that agressive.
I currently have two firemouths and a female con. one firemouth is 1" the other is 2" and the female con is about 1.5". They have worked out a pecking order according to thier size. I want to take them out of my 100 gallon and put them in my 55 gallon. What do you think about without the catfish?
I would go with the cons or the firemouths and the catfish. Be wary if they breed though because catfish are always hungry! I have seen convicts take over a ninety gallon from JD, Oscar and Blackbelt. All other fish at least an inch longer.