The regal is almost certain to chase it in any scenario. The goal would be to minimize the initial aggression to occasional chasing. This is would persist indefinitely, but should be manageable if you can make it past the initial phase of aggression. I have had angels together in the same tank without major issues, but it’s harder in a smaller tank.
The easiest thing to do would be to add another angel at the same time, so the aggression would be spread out. This option would increase your bioload with another relatively larger fish. A flame angel would be able to hold its own. If you went with this option, I would cut some fish from that list for now, and access later. If it were me, I would scratch the clowns and the goby. I know people love clowns in reef tanks, but I haven’t kept them in my community reef setups in years because they often become aggressive and will often host and harass other corals even when provided with anemone.
You could try rearranging the rock work right before adding the other angel, but that may not work.
Placing the new angel in an acclimation box for 2-4 weeks is a good idea, but I would still recommend the second angel in case the regal doesn’t have the territorial aggression out of her system yet.
I’m not sure what corals you have/were planning for, but the more angels you have means more picking. If you have fleshy lps (ie acanthos, trachys, cynarina, acans, etc), the angels are extremely to pick at them until they die.