Both groups of wild festivums I've kept (M. acora and M. festivus) have been aggressive toward tankmates. The M. acora attacked my 6" discus when the festivums themselves were only about 2.5" in size. My M. festivus are currently living in a 150 gallon with four large Peruvian scalare angels and they have torn the angels' fins and pecked at their foreheads.
With both groups of festivums, I notice that they attack the forehead region of their tankmates where the body meets the first hard rays of the dorsal fin. They've done this consistently with angels, discus, and severums.
For now they seem to be doing okay together in the large tank because it gives the angels room to move out of the way. The festivums don't actively chase or seek out the angels to attack them, but if the angel is in close proximity the festivum will quickly dart and attack.
I originally bought the festivums because I heard they were peaceful community cichlids like keyholes, but I've actually found them more aggressive than the severums I've been keeping them with. They definitely dominate my 150 gallon tank and they're still only 3" or so.
HTH.
Ryan