When I was a newbe many moon ago, I had a Sailfin, Purple and Yellow in a 90 gal tank. They did fine for 2 years until I broke my tank down.
If you want, that combination is fine. They will be fine in your 250.
I hear ya. The red sea is awesome, too bad it and a purple would kill each other.
Powder Blue and Purple would be really nice. Of these two, the PB is more aggressive. I would add Purple first then Powder blue, or else add at the same time.
If you decided which tangs to add, we can give recommendation on the order
I have a 9" d Sailfin, a blue and a orange shoulder in my 210. They are all beautiful and get along. The orange shoulder has a crazy amount of personality and would defiantly worth looking into. FYI, while the orange shoulders can get quite large, mine has been the slowest growing of the tangs I've kept.