What sex JD, what sex severum? Males are much more unlikely to work in most cases. GT's can either be extremely aggressive or very placid, it's always a risk. Personally I wouldn't add anything else.
the severum is almost deffinitely a female but i don't know about the jd. he's a blue jack and i read that for some reason most blue jd's are male. who knows if it's true. i'm not sure i want to add anything either but my gf wants something we don't already have and that's the only thing i can think of that fits the bill and is tough enough to live with a jd
the tank should be left as is, the blue variety of JD is actually weaker than a normal JD their immune system is weaker and as a result any little bit of stress to that fish will result in it becoming sick. and throwing the dice and getting a GT would not only be risky in terms of their disposition, but also the stocking levels as stated before when those fish reach adult hood your tank will be maxed out