The first fish I purchased for my 46bow included 4 2" damsels (three blue and one striped) and a 3" niger trigger. Well, the first day was fine, but the second day was a different story. The niger killed all three of the blue damsels and was chasing the stiped one around the tank. He could not catch the stripped damsel and after a week, they were co-existing with no problems.
In summary, from my experience, it is a crap-shoot. The niger is one of the most docile of the triggers but is still a predator. With fish of similar size and aggression, things will probably be fine, but the niger will grow to 3 or 4 times the size of those fish and if he can catch them, he will kill them.