I used to keep 5 hippo tangs in my 150G, and I thought they are schoolers. They were, for about a year or so. Gradually, as they get larger, the weak ones got singled out and I have to start salvaging them out of the tank to my LFS. Pretty soon, all there is remaining the the dominant one. If I have to do it again, 150G is too small for school of tang IMO. I would go with at least 300 or 400+ gallon tanks.
However, different species of tangs do school together. I currently have 1 hippo, 1 purple, and 1 jewel tang. They do swim together..................well, the tank this small, it is hard to see them NOT swimming together.
The pecking order activities are more obvious when you have one or more of the same species of tang. Whereas if you have multiple species of tang (and especially if their feeding requirement is all different), the aggression among themselves is much less common.