scott: I've never kept festae, but it would seem to me that, given their larger size, they would eventually just kill the convict. Perhaps the generous dimensions of your tank would give the convict room to run, but as another cichlid it would be disinclined to take that option and that's where the mayhem would be likely to begin.
Here are a few other thoughts as to what might work in your tank:
01. A school of 5 good-sized silver dollars might do well. Your tank gives them enough room to run. I keep silver dollars in with all of my large cichlids, and have yet to lose one to cichlid predation.
02. Or, you could protect your convict with a tank divider. Drawback: somewhat unsightly.
03. Or, you could try putting a mirror at the far end of the tank.
04. Or, a large tank adjacent with another pair of something else in there.
05. Or, a few large synodontis (decorus comes to mind). Just give them some lengths of pipe to retreat to, and they would be fine.
In general, however, I will just say that the larger the tank, the more things are likely to work out without any aggression at all. My 400 has two breeding pairs of Vieja synspila, a large Amphilophus citrinellum, four adult pacus, two large Synodontis angelicus, one Distichodus sexfasciatus, a Jack Dempsey, a marbled sailfin pleco, and a Hypselacara temporalis, and although everyone jockeys for position, there are never any serious fights.