I feel like the size of a Tang relative to the other dominant fish in the tank and the order they are introduced makes a big difference. I have kept small Achilles that were very shy at first and needed another active fish that could help draw them out and make them feel safe. There’s something about other Tangs that little Achilles seem to like and they buddy up a lot of times. I’ve seen the same thing out of Purple Tangs as well. Now, an Achilles that’s bigger than the size of your palm who has been plucked off the reef, shipped in darkness, and then kept with 20 other Achilles for a few days, packed up and shipped again to be put into a box at your LFS all by themselves for however long, then they enter your crowded classroom? That recipe occasionally results in an Achilles that decides to themselves, “When I get to prison, I’m shanking everyone.”
If you get to observe the Achilles or any other fish you want, take the time to check out their temperament. Are they overly skittish or hide and won’t come out? You should see there’s a natural curiosity from them. They’re not dogs, but there’s an intelligence and individual personality Tangs have. They insist that you notice them. They react differently to different people. They know when you’re mad at them. An Achilles isn’t as personable as a Desjardini or how a Vlamingii can be as adorable as a puppy, but If you have excellent water quality and the kind of flow in areas where you can see they have to work that tail to get around and graze, you can do it and in a tank that size, it should be just fine. You could also take a look at a Gold Rim which is one of my favorite Tangs and usually well behaved. I don’t remember ever hearing Gold Rim horror story.
My worthless opinion is: I say skip the Unicorn, Black Tangs are awesome, pretty peaceful and should be added early. Gems are cool but a Purple Tang, Dussumier or Maculiceps are prettier, Naso always seem to be pretty stable and should be added early. Just be prepared and have a plan to trap a problem fish in place before there’s possibly one who is picked on or a bully that needs a timeout.