Regarding the tangs, I found some information in Thresher,
Reproduction in Reef Fishes.
Nothing specifically on purple tangs, but lots of information. He cites two sources as saying that "temporary spawning colors they observed were identical to those developed by the males during agonistic encounters" (p. 274).
As far as sexing and size dimorphism in Zebrasoma, in one Zebrasoma species males were larger than females, in another species females were larger than males (p. 273). No help there.
Courtship behaviors:
Though details are still largely lacking, general spawning behavior appears to be similar throughout these fishes. The male approaches the female, may develop a distinctive spawning color, and then moves parallel to her. In A. dussumieri the male also rolls dorsally toward the female while lying on the bottom before her and vibrates his body vigorously while moving across in front of her (p. 274).
The pair then make a spawning ascent.
As far as length of courtship:
At first ... groups tend to form and break up rapidly, but as darkness approaches they last increasingly longer and the movements of the fish increase in speed (p. 275).
Hmm. That's a lot like what I saw but there was no spawning ascent.
A great book if you ever see it.
Jason--I think there's going to have to be some live rock in the display. As it is, I don't think there'll be enough shelter sites for fish until there's a lot of coral growth. So, I'll build some homes for every one before I toss any fish in there.