You could try removing all/most of the décor out of your tank, by removing the bogwood / rocks the fish will have nowhere to defend as their territory. The severum should be able to hold its own with the other two fish mentioned. When young severum can be misleading as they are very peaceful fish but as they grow that changes. In my experience there is very little aggression with south American cichlids, most of it is just show, it never leads to any fish being killed or injured. Note I am only referring to south American cichlids here not the Central American Cichlids which tend to be a lot bigger and often do not mix well in a community tank set up for cichlids.