Flame Angels are dimorhpic so you can tell a male from female/juvenile. You can add a female to your tank and she will be fine with the establish fish. Two females or male and female will be fine just don't put two males in that small of a tank. They will kill each other.
Make sure the new fish is eating well and healthy. She may get chase and nipped a little from the establish fish (male or female) but it will be fine after a few days. If you add a male on male, there will be a hell bend to exterminated. In larger tank with plenty of rock, the weaker male will eventually turn back to be a female, but not in confined space of a 72 gal tank.
120 gal with a lot of rock is the minimal I have heard of male reverted back to female.
I have a pair of Flame angels in my 65 gal tank at different times. They spawn all the time. I got a video of them online somewhere.
Regard tangs, Sailfin does grow very fast. A small Sailfin Tang in a 72 gal tank is stunned tang, but I guess one can argue that none of our aquarium tang reach their potential size in the wild mean that they are all stunned tangs. I had Sailfin tang when I first keep reef aquarium but have not have one in 30 years. IMO, they get too huge, like the Naso tangs.
My 65 gal tank also have a PBT that was emaciated and barely 2 inches, full of ich when I put him in there. He grew to over 6 inches and was re-homed in my 320 gal home tank in about 6-7 months. When I removed him from my 65 gal tank, he was a picture of health and fat, far cry from when he first went in, arrived from LiveAquaria.