Tank dims: 96L x 30H x 24W
I have a Vlamingii, kole, sailfin, PURPLE, 5 yellows, 2 scopas and 3 hippos. None of the tangs give me any trouble, even the purple, becase they have to be introduced properly and into a properly designed tank for them. The only rule of thumb for purple tangs I have not personally disproven is that if you want multiples you have to go three or more because they do no do well in pairs. I have had between three and five at any given time but never a pair. Tas5tas was right about them liking more swimming room so 3' is kind of limited for your tangs, but none of the ones you mentioned get that big (at least not in a home aquarium).
As far as stocking goes, from what I've seen, if you want an SPS dominant tank you really need to limit your fish selection. They put off alot of waste so unless you're skimming the heck out of your tank, running carbon, a phosban reactor and you're religious about your water changes you probably want to stick with three or four small fishes for a tank that size. That's just my opinion but you can check with some of the other guys who run SPS only and see what they say.
As far as dimensions go those are great dimensions but you might have a problem reaching the middle. From the top center of any panel to the middle of the bottom of the tank you are looking at 2'-1.5", even more if you're trying to reach it from a corner. Unless you plan on doing a very deep sand bed I wouldn't go more than 24" because reaching the bottom becomes a limiting factor, and you're just not that tall Allen.