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.
No, they weren't cheap, but as a discus keeper they didn't seem too expensive Besides, they were worth it. They are nothing like wild-caughts. They act just like tank-raised U. amphiacanthoides -- devour everything, very active, extremely healthy. Hopefully in a year or so I'll have fry for everyone.
I've only ever seen them through Rapps in more than single specimins for sale, and that $120 price tag made me swoon, even after breeding discus for a while. Haha.
Hopefully you've got some breeding shots up in a year or two, I know Uaru are pretty much the hardest commonly available SA to breed, period. Though I don't know if Uaru F. are any different from Uaru A., if anyone's got a chance at getting them to spawn it's a discus keeper