Clarence,
In my humble opinion, you are asking for trouble, here's why I think that.
You have a 55 gal system housing, Zebrasoma flavescens, Paracanthurus hepatus, Acanthurus leucosternon and a few other critters. It's the 3 Tangs that are going to cause problems. If you are getting by with these 3 Tangs without problems you are fortunate. Adding a fourth 6" Tang of unknown species (at this time) there is going to be territorial terror. We know that Tangs are notoriously susceptible to Ich often caused by stress. Adding the new Tang is likely to cause stress. Ich is almost always present in your aquarium and the new comer could bring it in, too.
Adding just one more may be the straw that breaks the camel's back and you pay a heavy price in losses. I wouldn't do it. But, you need to make that decision. At $200 it's a bit pricy for what it looks like in the picture.
Just my opinion.
Dick
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