You are getting your info from the wrong places. And the "You can keep this fish in this tank for a while" is not smart thinking. It's not as simple as just pulling the fish out and handing it over to your LFS. Trapping and catching fish is NOT an exact science and each transfer is very stressful on the fish. A 30 gallon tank is essentially a holding tank/quarantine tank for (ie a few months at most) for ANY Tang. Beyond that and the chances of success diminish rapidly. As for the Dwarf Angel, that is incorrect as well. Dwarf Angels are best suited for tanks that are 4 foot long or more. Take it from a guy who tried a Flame Angel in a 36 gallon bowfront (30 inch long tank) and had it blow up in his face. Fish was a complete butthead. If he had a 3 foot long tank I might feel comfortable recommending a Cherubfish (aka Pygmy Angel) but that's as close as I would get to an Angel in his tank. It's always best practice to buy a fish that can do well at it's max size in the tank that you have.
Oh and for Yellow Tangs, I wouldn't even go with a 4 foot tank, I would say 5 foot minimum. This coming from a guy who had a 4 inch Yellow Tang take over his entire 90 gallon after just one month.