The purple tangs will kill each other, they are really aggressive tangs, maybe the most aggressive tang of the zebrasoma genus. Regal angels are hard fish to keep and can be very picky eaters, I would only attempt one if you have experience with fish that are tricky to get eating. Marine Bettas get pretty big and I have heard that they can be tricky to get eating, but you may want to talk to someone who has owned one and see what they deal is. A single purple tang and a majestic angel might make a nice pair, just make sure to add the majestic first and give him a chance to settle in for a while before adding the tang.