the yellow may fight the regal if he is established in your tank, best to add tangs at the same time. you can try with some risk, change rock work layout and cut of lights for several hours just before adding the regal. would may have been easier if the regal was added first as the yellow is be somewhat more territorial. i have the 2 in my tank and the regal was in 1st and he also was aggressive towards the yellow for 2 weeks. had to use a divider for 1 week and they finally accepted one another. risky, so good luck! tangs mix alot easier in lg tanks 150g+