The first Niger I owned got to be overly agressive and at the time, I had some fish that couldn't handle it. Had to trade the poor guy in. Then a year later, I got another Niger, this time he has much better suited (agressive) tank mates and he's been really behaving, not that he has a choice. He's in with a much larger goldheart trigger which noone messes with, a larger sargassum trigger, a queen angel which again noone messes with, a naso tang and a large foxface. So he really has noone to harass.
From my experience, I think Nigers are unpredictable and the bigger they get, the more agressive they become. Great for a tank with equally agressive fish. Seem like fast growers.