I have a 210 with a purple, yellow, and a sailfin. All of them were added at different times, they get along great. So i think this is inline with clown triggers, it is a crap shoot, or hit/miss. Good luck whatever you do decide to do.
If you have the money, it is worth it in my opinion. Why limit your tank to what everyone else has in their tank. I saw one of these BLACK TANGS at a LFS in Colorado and it was about 7 in. . WOW is all I can say. It is always nice to have a rare fish in your ST. I say go for it .
What you are experiencing is very normal on a day to day basis. My puffer likes to lay in the very front right corner of my tank. He usually does this every other day. He just lays right on top of the sandbed. Don't be alarmed this is normal behavior. Good luck with your puffer, they are...