Three pairs of angelfish. So a total of 6 angelfish?
I think that as long as you can divide up the tank into many different "Sections" where one pair of angelfish can't see the other pair while they are breeding I think that you could do it in a 500 Gallon tank, maybe. That's not to say the the different pairs won't have any confruntation in there search for a breeding spot. IMO.
I've never done more than 1 pair of angelfish in one tank, but it would be fun to try sometime.
If you are not planning on raising the babies, only watching the parents care for them you could try and make it a planted tank with swords and different anubias species with a much of slower moving fish so the parents could defend the babies. I think that would be AMAZING!
What ever you decide on doing make sure that you post lots of updates on your pregress.
I think this could be very interesting.