I would worry more about the Mesonauta than the severum, just based on my experiences. I've raised and spawned two different groups of wild Mesonauta, two different species, and they were very nippy when they got to about 6" or so. They loved to attack my severums from above. It was never too violent, just nips at their forehead and dorsal fins, but it did make my severums jumpy.
IMO, 55 gallon is too small for these fish for life. Festivum males grow large, and being shoaling fish, they really do best in a group of 4 - 6 or more (if tank size allows for it). Either way, they do end up larger...
This guy was over 7" when I had to re-home him for shredding severums' fins in my 210 gallon.