I have kept a school of 8-9 yellows in my tank for the past 7 years. I also have a purple, achilles, chevron and 2 large Hippos.
In smaller numbers they can get nasty as one is always the big fish. When you have a bigger group there is a lot less bullying. I like to think that there is peace within the chaos.
There are no territory issues, the yellow tangs are all over the tank. There are usually 2-4 in the middle open section of the tank.
When the lights are ramping up in the evening I kill all the flow in the tank and only the return is on. During that time the yellow tangs and the purple usually school up and go picking food from the rock work in the tank in a nice compact little group.
From my existing group 2 have been in the tank for 10+ years. 3-4 for the past seven years and 3-4 were added after a tank crash 2.5 years ago that wiped out part of the school. When adding to the existing fish population there was maybe a 24 hour period of the existing fish establishing dominance and then everybody was fine.
I am also a lover of small fish, so whenever I have added tangs to the group I always add smaller fish (not usually recommended)
Dave B