The clown tang is about 3" and has been that size for 3 years now.
I don't know why it has stayed so small, but I'd rather it stay that way.
I do have a cole tang the same size and a yellow tang the size of a quarter.
They are all fine together, but the clown was last to be added.
As far as adding any more, I wouldn't do it.
A 210 is not large enough, plus to answer your question, the clown can get very aggressive with a new fish.
If and when it starts to grow again it will go to a friends 650gal.
A new fish can not be even close to the same shape as the clown or it wil be killed.
The clown in my tank, like the seahorse was not eating at the LFS and I took a chance, and figured my tank was better than a 20gal full of copper.
Kedd