For Fantasmic, you should try and book a Fantasmic Dinner Package (FDP). This can be booked at Mama Melrose, Hollywood and Vine and The Brown Derby. There is a price per person charge and I believe that you need a credit card to guarantee the package. I also believe that you will be charged if you "no show". The benefit of the package is that you will receive a special pass that lets you into the back entrance (by the stroller rental in the front of MGM) and into reserved seating for FDP patrons. This eliminates the necessity to arrive 1 1/2 - 2 hours before the show. You need to arrive about 30 - 40 minutes prior with the package.
I would abslolutely try and secure an ADR for 50's Primetime. On both of our last trips, they were turning away anyone without an ADR. Hope that helps.