My six wild festivums dominated a 55 gallon at 1.5" in size. They chased and attacked adult discus that were bigger than my hand. I think it had a lot to do with the fact that the discus were too passive to chase back.
The fish are now 3" or so in a 150 gallon. They're currently sharing their home with
Uaru fernandezyepezi and I haven't seen any problems. The uaru are peaceful fish but they're also active and boisterous at feeding time and it seems to keep the festivums at bay. They mostly ignore each other but occasionally intermingle. I've only seen peaceful exchanges. There hasn't been any chasing or nipping.
My guess is that festivums would do well with larger wild angels which tend to be more aggressive than tank-bred angels. I've seen pictures of large festivums in tanks with adult angels and discus. My festivums seem to be opportunists, so don't keep them with anything too peaceful and they shouldn't take advantage of it. Every cichlid is different, though.
Oh, another word of caution -- festivums will kill and eat small fish. They polished off my ABN plecos and I know in the wild they eat things like cardinals.
Here's a couple of pictures of the festivums and uaru together. I know the tank is boring, but it's only a grow-out tank and they'll eventually have sand and driftwood in there.
http://i77.photobucket.com/albums/j49/r ... 04928e.jpg
http://i77.photobucket.com/albums/j49/r ... eeab1f.jpg