All very chill fish! I currently have my juvie discus with a juvie green severum and two juvie viejas. Only thing about discus is that they're not only chill, they're super shy so they can't tolerate aggression very well so you gotta be careful bout their tank mates.
I've got 2 6" green severums and I've never had a problem. My 8" Jack has proven to be pretty docile too but I've heard that they can get pretty aggressive sometimes