First off, I want to say that you could not pay me to place any fish in any of my displays that has not been QTed for at least one month. With that said your plan looks solid... if the new regal was fully QTed and adapted to captivity before placing it in the display, the divider could be lifted at one month.
Remove the divider on a day you have off where you could just relax around the house. Leave only ambient lighting on for the first few days. You will know within a few hours how easy the process will be, but if you follow the detailed steps you should be fine. With healthy adapted regals they could take a beating from each other to establish dominance, so do not worry. Once a dominant fish is established the aggression will always be there, even in spawning pairs, but it is natural... the subservient fish will always be aware of where the dominant fish is and deal with it fine. Interestingly, as is rather common, with my trio the two larger individuals gave each other more problems than either did to the small specimen... regals are tough fish though once adapted... notice one of my small Marshall Islands girls (my trio is from the Indian Ocean) in with the rough and tumble large angels in the pic above! As I always say... it's not WHAT you mix, but HOW you mix!
In all honesty I would be more worried about the achilles than the existing regal... I love these fish... I own an achilles/goldrim hybrid and a goldrim/powder blue hybrid, but their initial aggression makes me sometimes want to shoot them with a mini spear gun... be prepared...
Copps