Epaulettes really aren't fish eaters, but rather crustacean hunters.
Vlamingi and nasos really shouldn't have much aggression. Use a social acclimation box to introduce a new one to an established one.
A degree of aggression is normal if there is going to be more than one tang in a tank. Though having a big vlamingi or naso helps limit that, as they are a gentle giant for tangs, and can be at the top of the hierarchy without causing too much drama. High flow and frequent feedings also mitigate aggression among tangs.
Stress itself isn't a factor for HLLE. Carbon, stray current, diet, and water quality are all bigger contributors.
