I would add it last(if possible at the same time). If you just have to have a the Achilles first, buy it small as you can. Then purchase the other Tangs bigger, so they will have a chance. Also use a acclimation box to see how they will react.
Indeed,when his eyes turn blue during aggresion times you can reeeeally see his anger:eek1:Do you have an Achilles? Size doesn't necessarily equate to subordination or being less dominant. I had a larger Achilles, didn't develop enough of an attitude to become a bully, maybe never would have. It died so I don't know. I have a small Achilles now, and trust me it doesn't care if a fish is bigger than it or not. The eyes don't lie on this fish.
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Before my last 450g broke I had tons of 'aggressive' tangs and they all got along quite well. Perhaps it is the size of the tank that keeps them docile. They were always doing the pecking order game but never transitioned into a fight (to clarify, other than the first day that I would add a new tang... those days were ugly).
Regardless, I am passing on an AT as first fish.
I would add the Achilles and powder blue at the same time, closer to the end of your first adding, expecially tangs. I have mine in a 300 and the Achilles and powder blue still dance during light dusk period. Otherwise, no issues