The Tang will be the more territorial fish for sure so adding after the Copperbanded first should be no trouble. I had a rather large Black Tang in my tank for 2+ years and added a smaller Muelleri butterfly a few months back. The tang gave the Muelleri the "what for" for a few days...it was pretty intense, that was after I had the Muelleri in an acclimation box inside the tank for 5 days so they could get use to each other. After the first week things were fine and the Muelleri and Tang co-existed. Within a month of that I found the Black Tang a bigger home. The tank has been without the Tang for about 3 weeks and I just added a small juvenile black tang back into the mix, They are so beautiful and good worker fish around the tank keeping any macro algae from getting a hold. You should see what sprouted in the weeks since the Tang exited the tank, I had no idea how much my Tang was taking of biz!!! The Muelleri gave it a few good pokes, some minor chasing and posturing but for the most part it was minor territorial welcoming. The new tang is much smaller than the Muelleri and maybe the Muelleri remembers the past, hehehe. I say get that Copperbanded in there and add the tang afterward.