Get them small like 3'' will give you a couple of years before they outgrow your system.Its the Sailfin that grows huge(15'')not the Powder Blue(9'').None the less,its the swimming space and not the actual size of the fish.Purple tang is fine,add it last or at the same time as the other tang.They can be mean,aggressive boogers.
You could always go with a tang from the Ctenochaetus family.They don't grow as large,usually max out at 6-7''.The Yellow Eye or Blue Eye Kole and Bristletooth Tomili comes to mine.