Well, first off, measure the tank's dimensions, a 65g is normally 36". This will make a big difference in recommendations. Going by the full tank shot picture, it looks 36", but I could be wrong.FourSeamer said:
The mystery fish is a severum (Heros efasciatus), which strain or collection point, I don't know, not my forte. They're a South American fish. Doesn't really go with the Malawi, also, they can reach around 12", so at some point, it will be 6" smaller than the width of the tank.
You want to leave the barbs...to my knowledge they're usually hit or miss, more often than not ok, depending a lot on what they're housed with. So if you're looking to add to the tank, I would re-home the Severum and look for another calm, passive species, perhaps a group of Rusties (Iodotropheus Sprengerae). I'd be worried about the barbs by adding anything on the more aggressive side. As it is, I'm not sure they'd be fine with Rusties or not.
If it is indeed a 36" tank, I wouldn't add anything, I'd re-home the severum and seriously consider upgrading the tank if you want to keep the acei and azureus, as they should be in a bare minumum 4' 55g, at best at least a 75g.