Red Tails can be unpredictable... Some are ok in communities, others just will not tolerate intruders. It sounds like you got one of the more ornery ones. It's possible he will settle down once he gets through puberty. My Red Tail pestered everyone in the tank for a couple months once it hit puberty, but these days it just hangs out under it's tree trunk most of the day.
Adding extra hidey holes may help. My shark couldn't seem to decide between which of three decorations it wanted to claim as its own during it's "teenager" stage, but after a few tank rescapes and some time to mature, it finally settled on the tree trunk. Having extra places for it to pick also gives the Severum a place to hide in when the shark is being moody.
When all's said and done, you still may want to consider rehoming the shark if you're worried that the Severum won't make it.
