I have had a few copperbands with yellow tangs. The tang will show aggression for the first couple hours or half day maybe. After that it’s no issue at all. But they are all relatively close in size. Might be different if the YT is significantly bigger.
regarding corals - the 3 CBB I have had were model citizens for 3 - 6 months and then either started picking at my baby rock flower nems (always leaves the big ones alone) or my clams so they had to go. I am on number 4 now. I got the smallest one I could find hoping that it had never tasted a clam before.
one thing to note is that every CBB I have had will destroy the small aiptasia but none will eat the bigger aiptasia greater than 1.5 inches or so in diameter. I still have to use F-Aiptasia on those. The F-aiptasia worked very well.