Yes, Fantasmic is at MGM, it runs nightly during low season and twice nightly during peak season. Yes it is at night, outdoors, so it is subject to weather-related cancellations. No, this is not a fast pass attraction. But there is a "Fantasmic dinner package" that you can book if you are concerned about not getting seating. From all I have read (and seen) the "reserved" seats are not in the very best area, but most seats in the theater are pretty good. It requires a priority seating reservation at one of three restaurants, Mama Melrose, Hollywood and Vine and I believe the Hollywood Brown Derby. After your meal you get a voucher for admission into the "reserved" section. But this section can fill up also, so you still must arrive early!
During peak season I believe for good seats it is necessary to arrive 90 minutes before the show.... we have shown up during low season within 45 minutes to 30 minutes before and gotten great seats. There are snack vendors inside the stadium, so it is possible to get some dinner or a snack (pretzels, popcorn, hotdogs, soda) while you wait.....................HTH.............P