Similar thought here. Tank size makes a difference, sometimes a dramatic difference, in severum behavior. They don't like to be crowded by tank dimensions imo. I've taken what was a psycho killer severum off someone's hands and simply put it in a larger tank and had the same fish be quite peaceful. I've seen people try to breed severum pairs in 29 gal 'breeder tanks' and they can't figure out why the same fish that paired up in the display tank now want to kill each other. A 55 is tight, would depend on the particular individuals, including individual temperament and size, possibly aquascape would make a difference, but imo it's a might work, might not scenario (sans the angelfish).
I've kept angels and severums together, including both severum and angelfish pairs, with little problem at all-- but in a much bigger tank.