I have 4 tangs in my 180 (Sailfin, Yellow, Purple and Blue Hippo) and they are all doing well and thriving. The purple is the most aggressive of my four. However, he and the yellow tang are the ones usually going at it. With that being said, he is usually (~90%+) fine and I have never seen him bother any of my other fish.
Like Jake said, I would put him in last and if you are getting 2 tangs, I would suggest that you get two with differnt body types, like a purple and a hippo for instance and I highly suggest you put them in the tank at the same time.
Good Luck,
John