I can only tell you that this is the way it was done last year. We made the PS when we ordered the tickets. The tickets you pay for in advance, the PS you do NOT.
I would highly recommend doing it this way, otherwise, they usually book up. Now, I understand they opened some more times at the 90 day mark, but I would not take a chance and who knows if all of the good times will be taken.
I also recommend getting as early a PS as possible so that you can get the most out of MNSSHP. Eat first, then shuttle over to Tomorrowland, ride all of those rides and a few minutes before the party, make your way into Fantasyland. You will find that by that time most of these rides are open as well and you can get these under your belt so that you can do the things that are only available during the party, like Trick or Treating, the parade and the storytelling at Liberty Square. Also, get your picture taken early to get it out of the way!
Hope this helps!!