Not sure if it is any help. But I have Two yellow tangs and a scopas tang in a 180. It was a rough start for the scopas. (last one added) If you can use a transparent divider it will help. It may also help if the fish are of different sizes. As our larger yellow seemed less annoyed with the scopas ,compared to the yellow that is the same size as the scopas.
If all the tangs are on the small side, yes you get away with that many in 210. Since you already have a purple tang I would go for two or three small yellow tangs. even though they are smaller they will ban together and intimidate the purple tang. if you want one yellow go with one hat is larger than the purple. But anything can happen.
tangs that are existing in the tank seem to be very aggressive to anything that gets in its territory that makes it feel threatened. my yellow tang was trying to cut me with its blades today when i had my hands in the tank. i dont think size will matter too much. so i like flynn said, anything can happen.
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