The only thing I'd be worried about would be the JD and the Severum having territorial issues. Kinda fish-dependent and also depends on how you set up the tank. As far as adding anything else, I'd add some dither fish to keep everyone out and confident, and also to help distract the JD and Severum from going at each other, plus they'd add a lot of movement to what otherwise would be a pretty slow-moving tank. Giant Danios or Congo Tetras work well for these. I'd say Silver Dollars too, but in order to get a decent sized school I think you'd be pushing your bio-load.
I tend to be a bit overstocked with my tanks though, and that means I put in more time in tank maintenance than you might be willing to do. Your filtration is also a big part of this. If it's not up to par, you're going to have issues with even the stock you already have.