The tang issue is probably the most debated thing in the hobby. I once saw a post by some guy from Singapore with 5 tangs in a 65 gallon aquarium, and there are some people who think tangs should NEVER be kept in a tank of any size under any circumstances.
IMO, the bigger the better for tangs. Why settle for the minimum recommended size? Yellow tangs, for example, grow HUGE in 1000 gallon tanks, but are stunted in most tanks, like 40-120. Tangs really love to swim, and since most other fish we keep don't present such space demands, most people don't have great big tanks.
I had a yellow in my 70. He was fat and healthy but didn't seem happy in there. I also wanted to grow caulerpa, so I took him back to the LFS.
We talk so much about keeping animals healthy, but what about keeping them happy??? Sure, an elephant might be healthy on 20 acres with a watering hole and *lots* of food, but would it be happy? The same goes for a 3 ton orca at a public aquarium, or a polar bear in a zoo.
I think a large indoor pond would make more sense for tangs than a rectangular aquarium. Maybe an 8' diameter circle 3' deep. I still wouldn't put certain species in there.