Hi there, Victoria!
It's great that you are considering all of these issues in advance. A bit of pre-planning can definitely make the trip to Walt Disney World run smoothly!
Fantasmic! is approximately 26 minutes long, so you are already nudging up against your reservation time.
The theater is located in the Hollywood Hills Amphitheater, which is adjacent to the Tower of Terror.
The
Sci-Fi Dine-In Theater is roughly next to Star Tours on the opposite side of the Park.
Accounting for crowds at exit time and the duration of the walk, it's my opinion that it would be very difficult for you to make that 7:30 seating.
Typically, your reservation will be held for 15 minutes after your seating time. If you call the restaurant via (407) WDW-DINE en route to your meal and explain your situation, you can ask if your reservation will be held a bit longer. The Cast Member does not have to hold the reservation, but often they do their best to accommodate their diners.
Please bear in mind that if you made these reservations after October 11, 2011, and were asked for a credit card number, you will be charged $10 per name on the reservation if you are unable to keep it. The cancellation policy required 24 hours advance notice.
It might ease your mind to try to get a slightly later seating time to alleviate some of the pressure.
Give (407) WDW-DINE a call and see what magic they can bring to you.
See you later, celebrator,
-Deborah B.