Many breeders keep breeding pairs in 20 highs 24"Lx12"Wx16"D, not 20 longs, 30"Lx12"Wx12"D. I've heard of people keeping pairs in 15 highs, 20"Lx12"Wx18"D.
I've kept standard fin pairs in 20 highs, veils seem to need a little more room, 29's are better, 30"Lx12"Wx18"D. 20's are 16" tall, a veil can get 12" tall if you include the fins. This doesn't leave a lot of vertical room.
29's are 18" tall, leaving a little more room to swim & grow. You can also keep a school of corys & a small plec in with 2 angels in a 29, you really have to keep an eye on water parameters to do this in a 20.
Fully grown angels really need 10 gallons each, you can keep potential breeders in 5 gallons each, angels tend to pair up before reaching maximim size. By doing this, you can get 2 compatable fish, just make sure you have someplace to rehome the other 2, and keep a divider you know works right next to the tank.
Tolak