I've kept two Regal Angels.
First one lasted a year and I lost it when my Ritteri wandered into a powerhead and shredded the tentacles, releasing nematocysts killing every fish in the tank except the clowns.
The second I just recently lost to an ich outbreak and my bungled treatment response. I'd had it for right at 7 years.
In my experience with both of those angels, Regals are not smart.
They take about a year in captivity to become aggressive feeders and it takes them a couple of months to recognize food items in the aquarium. This was helped along by a lengthy period in QT with dither fish who are non aggressive and non boisterous but are active feeders in the aquarium.
Big, established Tangs that are dominant personalities are not good for bringing in a shy fish like that initially.
Might not be a bad idea to find another Regal, QT it with something like a smaller Tang, (I've used Ctenochaetus tangs for this in the past) or something else that will take Nori off of a clip and feed from the water column....and condition it for several months in a protected environment.
Once the Regal is more acclimated and feeding regularly, the fun begins....
Removing the aggressive tangs from your display.
If you could move the aggressive/dominant tangs to a QT set up for a few months and give the Regal a chance to get acclimated and comfortable in the bigger tank, then you could re-introduce the dominant aggressive tangs to your display.
Just a suggestion.
It's alot of work....but I do miss my Regals....