Let me start by saying that the temperament of cichlids can vary widely from fish to fish. However, I personally would never try keeping my severum with angelfish or gouramis. They tend to grow very quickly as juveniles, as you have seen, and will become increasingly aggressive with age. They can reach 8-10 inches as adults. Also, I don't think that a 55 gallon is large enough to house 2 severums at adult size with all the other fish you have. Even the 2 severums alone would likely pose a problem, as they will have to compete for territory. If you don't have a pair, one will probably not make it. These aren't cichlids that are known for aggression, but they are cichlids all the same. If you can either return them or move them to another tank, I would suggest doing so.
*edit* I almost forgot to welcome you to AA, lion!
Stick around, I think you'll like it here.