Did you find out if that was ich? Definitely looks like it could be. PBT are so prone to it that i think if someone says ich they catch it. lol
Back to your originally question i think the tangs you want are fine. I had all mine QT and put in small and most at the same time. I did already have purple, scopas, and sailfin. I have a 400 gal 8x36x27 with plenty of swim room and hideouts.
Tang list: 5 yellow tangs, 1 powder blue, 1 purple, 1 scopas, 1 flame fin tomini.
Most may say that's two many but i have no issues and all are fat and happy. I have seen one chase another around but never bad and usually the one being chased will then chase back, almost like a game. I've never seen one torn fin or scratch on any of my tangs. I do keep them well fed.