I agree that you need to choose your tankmates accordingly. Nigers CAN get mean when they hit about six inches. Some people say they are "nice" triggers. IME that isn't so. Heck even my bluethroat acts like a trigger once in awhile and beats the crap out of the wrasse.
Triggers are triggers. Don't be surprised when your niger has your nice Aussie Harlequin Tusk pinned in the back corner afraid to come out. :uzi: Been there, done that. Several times.