I figured the fish was in the best of health before you introduced him to your tank.
You're not one to do things half@$$ed.....
But I'm not at all surprised to see that the Tangs bullied it.
IME, Regals take right about a year in captivity to get fully acclimated and become comfortable enough to wade into the fray and eat aggressively.
I would almost always suggest adding a Regal first to any set up just to give it the best possible chance.
If it was eating pellets, you had a good one, they are just too timid initially to survive boisterous tankmates.
I dont blame you for not wanting to get rid of any of your fish, you've had them all for quite awhile.
An adult Emperor should do fine, I just wonder about the Tangs getting nasty without one for a bit.....