Ok, first off, the name "Festivum" is wrong. Latin words, like all words in Romance languages, have "gender," and the gender of the specific name has to agree with that of the genus. When the fish was in Cichlasoma, they were C. festivum. When they were reclassified as Mesonauta, the specific name was changed from "festivum" to "festivus," to agree with Mesonauta. So, the correct name of this fish is the "Festivus" Cichlid. And, if you're having trouble keeping angels, this is a fish for the rest of us.
However, the name Festivus is even more confused. Most people have never seen a festivus. The fish that is imported into the US is almost always M. insignis. Apparently, the two species are pretty darned similar, though they occur in different regions. Almost all of the exports occur in regions that Insignis occurs in. But, I am letting my nerd flag fly a little too high, and running off on nomenclature. . . . We used "Festivum" as a common name, and really, that can apply to any member of the genus. . . . .
Apparently, at one point, the Festivus was thought to be fairly closely related to Pterophyllum. The two genera share a lot of common traits, and are often sympatric. Kullander recognised this when he resurrected the Mesonauta genus and described several new species.
Apparently, Festivus and Angelfish will form mixed species shoals in the wild. The angelfish will often inhabit the centre of the shoal, with the Festivus on the outside. Festivus are quite a bit more aggressive, robust, and faster. It's kind of like having a group of destroyers around your air craft carriers. The Festivus help to protect the angels, and gain a larger shoal size as a result. Very, very cool.
I wrote an article on this behaviour with some really cool photos back around 2000 or so -- I'll have to look and see if I can dig it up at some point. I think it was titled "Guardian Angelfish: Interspecies Protective Behaviour" or something like that.
JW