How fast do these tickets sell out the weekend before Halloween? I'm wondering if I can wait till next month or should I purchase them now? Can you add your Halloween tickets into your package for Disney if it's all paid for and pay them off?
The busiest party of the year (Halloween night) sold out in August last year, so waiting a month should be fine. You have to pay for the party tickets in full at the time of purchase, you can't add them to your package and pay them off.
Last year (2014) we went to MNNSHP the Sunday night before Halloween, the 26th, and purchased our tickets the Sunday before on the 18th. We only had booked the trip 28 days out to begin with so I didn't have a ton of time to plan. I kept an eye on how far in advance each night was selling out and Halloween was the only night sold out far in advance. Most other night were selling out a few days in advance, especially the Fri nights. We had a blast.