"My Yellow tang is killing my Copperband"
Is one of the most popular threads on saltwater fish forums. Even in larger tanks as mentioned, it does not work out some times. If you have access to another tank or somewhere you can put the yellow tang while the Copperband adjusts, this will help your chances.
450L/117gals is probably enough for now, assuming all the fish are smaller. However the Regal tang will eventually outgrow that size tank. I have no idea why something like 55gal was mentioned. That would be a nightmare.