Hi Rebecca,
I absolutely love
Mickey's Not-So-Scary Halloween Party! My family and I have attended the party many times and always enjoy it!
Disney FastPass+ service will not be active during the party. Lines for attractions tend to be a bit shorter during parties. We were able to visit several attractions, meet lots of characters, trick or treat, enjoy the parade and fireworks without any problem. The parade usually runs twice. We always took advantage of the early parade to ride some of our favorite rides. Then we easily found a place to sit for the later parade.
My best advice is to join in the Halloween spirit. Wear costumes, even if they're only Halloween themed t-shirts. One year the best we could do were some light-up pumpkin necklaces but it still helped us feel more festive!
Please let us know if we can answer any other questions for you!
Wishing you a magical, not-so-scary vacation,
Amy H.