I'm also in the same situation.
We are doing the Fantasmic Package on Dec. 18th. Our ADR time at Hollywood & Vine is 4:30pm and there is two shows for Fantasmic 7pm and 8:30om. I read somewhere that if you have the Fantasmic Package you are to view the 7pm show. Ok let's see if I can explain this.
The reason for getting the Fantasmic Package (FP) is to avoid the wait for the show because it's very popluar. (usually one show per night) But as with the case of Dec. 18th (extra magic night) there is two shows. The crowd for the later show is not big enought to nececitate someone purchasing the FP.
But, if you wanted to half way guarantee yourself a seat, you would have to purchase the FP and see the earlier show. Put it another way. The wait time for someone to see the first show with Fantasmic Package, and the wait time for someone to eat on their own and see the later show, is about the same.