Robert,
I had a niger in my tank prior to changing over to a reef. He did like to harass the snails that were in there from time to time but never casued any other problems with animals.
They can tend to be a remodeling fish though. If they can move it or remodel the setup to their liking they will. Mine love to burrow out an area of the substrate below his favorite rock. Great fish to watch with lots of personality. I doubt he'll outgrow a 150 very soon.