it could be a 2/1 tank: use a small section(S) of the tank to breed convicts its easy and u can use them as feeders. the other section(S) could be something more ecstatic.
I thoroughly enjoyed my Trimac pair... they did great in a 4' 120... If decorated correctly the 6' 125 would allow for a few pairs of convicts to add some life to the tank...
Dovii are very strong swimmers and need more than 6 feet to live happily. Its like keeping an Oscar in a 40 gallon, it can be done, but the fish isnt gonna be happy.
OP, Festae would work, any Parachromis sp. except Dovii, Carpinte, Cyano... Basically anything except Dovii and Umbee or other fish that reach similar size of those.
I really like having my Freddy and Jag pair together in my 125. gives em plenty of space and they are entertaining, esp watching them mate, move around punds of sand and rocks.