We went in October many years ago at about the same time as you are planning, and the weather was iffy for swimming. One day really hot, the next in the 70's so not so great for swimming. As for crowds, there wasn't much (except for Jungle Cruise... go figure!) and we did the entire Fantasyland in an hour. Of course that was many years ago, and they've added Mickey's Philharmagic, which is a wonderful show. They've also added the Food and Wine Festival crowd and the Halloween party at MK. So, I think there are more crowds than when I went. However, we went last year during "Jersey Week" in November and it was hot enough to swim every day! And the crowds are heavier that week and it was still managable. With Fastpast in the picture, I think you should have little problem managing crowds at that time of year.
Enjoy!