One thing to note I see in your post you say "purple tangs" as in more than one. Even with a 72 gal
tank personally I dont think you should have more than one in there.
Tangs need lots of swimmign room and can grow to several inches in lenght.
As far as fish I think you have a wide array to pick from. A maroon and a tang would be nice. Maybe a few schooling fish to go along with them.
Inverts, well with a tang I would say a pair of skunk cleaners would be a good addition to help keep the tang free of parasites.
Is there anything aside from the tang and the clown that you would classfy as "must haves"?