All fish are different. Some are accepting of other and some flat out do not like the company of other fish.
For example, my niger trigger has been in my tank for 4 years and is more docile than any other fish I have ever owned (my black clowns bite me more than he has). On the other hand, I tried to add a blueline trigger a couple of years back and the niger went nuts...it was like some reaction went off in him and he turned into the tazmanian devil. If I didnt pull the blueline, the niger would have killed it for sure.