I ordered the melichtys niger, and got an indicus, from a site that no longer exists, Fishy Business Aquatics. The hawaiian is the larger, more colorful melichtys niger, and as far as I can tell many companies cannot tell the difference. Melichtys indicus is the indian trigger, which is far smaller at max size. The difficulty is that when they're upset or stressed, they change color. The indicus turns brownish from its original black, the niger turns black from its original blue/black. The pinktail trigger is in the same genus as these two (Melichtys)
The hawaiian shares its species name with the niger triggerfish, but they are not closely related. The way I differentiate m. Niger from m. Indicus is that the indicus has a white tip on its tail fin which curves outwards, while the niger has no white tip and has it tail fin curve inwards like a niger trigger's tail.
All in all I think you should check with the source you're buying from, but I think it's far more likely that any of those listings would be the smaller indicus. I have never seen the m. niger in the trade, and most retailers don't realize they're two different fish. The m. indicus I have is, however, extremely docile, as far as triggers go, and much more appropriately sized for most aquariums (since max possible size is like 10 inches, but most stop growing probably in the ~8 inches range).
