I would not add angelfish. My experience with Mesonauta seems to differ from what I've read elsewhere on this forum, but I've raised out two wild groups and they've both been particularly aggressive and nippy. They love to attack other fish from above. When my severums, angels, uaru, or chocolate cichlid are eating and they swim under the festivums, the festivums dart from above and attack them on their face/forehead/hard dorsal rays. All of my larger fish show scars from them.
They have pretty much destroyed the dorsal fins on my Peruvian angels. I thought they might mellow out as they grew, but they've actually gotten worse.
Both groups have contained 6 fish, and though that does help take stress off a single fish, it doesn't really curb the aggression. The six of them still chase and dart at one another all day.
I was also told that wild festivums were difficult to spawn, but I got pairs from each of the groups and the fry were extremely easy to raise (along the same lines as angel fry).
My suggestion is a large tank with tankmates who can push back. For instance, my male festivums are in the 4 - 5" range now, and when I added a big male green severum (7") to the 150g tank earlier this week he was immediately attacked by the festivums who ripped up his dorsal and caudal fins. He's mending now but the festivums didn't hesitate; they went right for him.
so it's going to take a much larger group if it works at all...but very cool fish I should say 
