Hi Shelly,
Although you have things scheduled very tightly, I think you should be able to make your dining reservation in time and still be able to enjoy a leisurely meal.
Fantasmic! is a twenty-five minute show and Disney is very good about starting their shows on time. You will need to factor in a few extra minutes to walk from the open-air theater space on Sunset Boulevard to the
50's Prime Time Cafe, but it's a very short walk. You can certainly make it in five minutes.
Even though Disney's Hollywood Studios is scheduled to close at 7:30 pm that evening, the cast members at the Prime Time Cafe will not rush you through your meal. Enjoy your interaction with the cast members there. They are loads of fun! However, if you will be relying on Disney's complimentary bus transportation back to a Disney resort, I would try to conclude your meal and be out at the bus stop by 8:30 pm as these buses are only guaranteed to run for an hour after park closing.
Enjoy your day at Disney's Hollywood Studios!
Jill