Does it help to get a FastPass for the Fantasmic show?

The key is to get to Fantasmic early.  Arriving 30 minutes early for Fantasmic will most likely get you a seat, and 45-60 minutes early will usually get you a great seat. The answer to getting a FastPass+ for the Fantasmic show is probably no unless you are going to be there on a busy day. 

In addition, if you were to choose a different FastPass, once you use your initial 3 FastPasses you can select the 4th one. And after you use your 4th you can get a 5th, and so on. There is a chance that you could still get the Fantasmic FastPass later in the day after using other FastPasses. 

Generally, we don't recommend using FastPass for the nighttime shows for these reasons, but it is your choice of course.