I've had at least two tangs in a 20 long QT for 12 weeks. A powder blue and a tomini. I think I had a flame angel in the QT at the same time. The trick I found was to have strong water movement to give them some exercise.
Yeah, a 20g isn't ideal for a tang. It would definitely be unsuitable as a long term display tank. But you do what you have to do. QT is intended to be a temporary situation. I intentionally use a 20g because the potential price of medicating a larger tank, trying to catch the fish in a larger tank, and the physical space needed are all turn offs to me.
I had a 10 inch vlamingi in my tank, and a 9 inch hippo. In my opinion, I would never recommend anything smaller than a 6 ft tank. In fact, I often felt my 6 ft tank was too small. Too small for these two fish because of their size is what I mean. I had 7 tangs in my DT that got along great, and were fat and healthy.