Generally Niger triggers are pretty placid, I know that Fixit/Blue had a niger in their reef system for a while. They took it out because they bought a 200 gal tank for more aggressive fish. But dlight had a niger in his tank, but he did get mean. But I think that the reason is that he cycled the tank with the trigger, which is alright, but any other fish that you put into that tank will be 'tresspassers' in his territory. So, always put the trigger in last, it could be dangerous.
But then again there are always exceptions to the rule.
-Mark