If everything is fine and the fish are not being overly aggressive then I don't really see a problem. If it's tank space that you're worried about I'd say it's fine even if you have 2 big plecos and 8 fish in total.
the 2 12 inch plecos are a lot of bioload for the tank already and two large fish competing for the same space in the tank.
the issue with the convicts is not so much the size, as the potential aggression if and when they pair off and start spawning (presuming a male female mix)