Any fish that gets bigger then your other ones will always pose a threat to the small fish no matter what the temperament. If it can fit in its mouth, there's a good chance it will eat it.
The only way I know of to keep a big fish from eating smaller ones is to feed it 100% flake or pellet food. No blood worms or other live/frozen food. I've seen big oscars that were raised on pellets and flake food that won't even think of eating a small fish.
That being said, sevrums can be pretty nice to have in a tank. They are pretty docile (especially if its a single one). I had a redhead sevrum (heros sp. rotkiel) in an oddly stocked community tank and it was great with the rest of the fish and never hurt a soul.