It shouldn't be too bad crowd wise. I think they only do one showing during that time of year at 9p, but don't quote me on that...in the busier season I've seen them do two showings at times.
Anyhow, I can't speak for that time of year as I haven't been. I go in mid-August, and at that time the line for Fantasmic is rediculous! It fills up the whole promanade outside of the theater and winds all the way out the front gates (which are next to Beauty and the Beast) and sometimes even loops around to Tower of Terror or down Sunset Blvd as far as the bakery...just...nuts...
With respect to your ADR, as I recall they limit the package to certain reservation times. While the package is "free" with your meal (it consists of a voucher which you will give to the CM at the side entrance, where they will guide you to the reserved seating section), I am fairly confident you will need to eat before Fantasmic.
It's worth giving 407.WDW.DINE a call to double check, if it interests you. You'll need to do that anyway to book the package (though, I did notice they finally added it to the website recently...I'd still call).
Normally the kid and I do a late lunch at Brown Derby to get it, though this year we are going to give Mama Melrose a shot (I've never eaten there...so it's something new for both of us).
The parks calendar lists Fantasmic as playing @ 6:30 that night. My ADR @ Brown Derby is for 7:30 as I read that the show is 30 mins long. My original plan was under the assumption that FP+ would be in place by Dec and I would use one of my 3 on Fantasmic. One of the my others would go towards TSMM (as we have also never done that either). Also bear in mind that we will also be using our DDP for the meal, not sure if that impacts whether or not you can use the Fantasmic Dining package.