If you do go with the Black Tang, I would make sure the size difference is large between him and the yellow with the black being larger. Yellows can be feisty but Blacks can hold their own when they are larger. I've had blacks from the size of a U.S quarter coin to the size of small plate and they have very different attitudes at those different sizes. If necessary, you might need to isolate the yellow until the black gets settled and familiar with the new tank setup. ( can you tell I am really into black Tangs?
Having said all that, a nice medium sized naso or unicorn might make for a less aggressive environment upon introduction. A Kole ( yelloweye) will be a good algae scraper for ya as well and still looks nice as an adult (opposed to say a Chevron (IMO
) ) with a Tomini adding a little extra color than a Kole.
But if the bottom line question is which fish will be the least aggressive towards the EXISTING fish, I'd go with Naso or Unicorn. They are far less aggressive than circular shaped Tangs. ( and Blacks can be aggressive!!!!)
Hope this helps