Keep an eye on the green Severum and make sure he's not bullying the tank. A little bit of showing his dominance is normal but in my experience most Severums don't inflict any damage. I say most because I've had many Severums throughout the years that have gotten along with any fish they've shared a tank with but I did have 1 green severum that was a complete nightmare. He spent all day harassing every fish in his tank, they all had torn fins. I took him back to the store where i got him and they let me swap him for another one. The new one got along with everything in the tank. As with any type of fish, there's always a chance you get an overly aggressive one.