Hi, Abbie!
On February 21st, Fantasmic is at 7 p.m., and will run about 25 minutes long. It will take quite awhile to get out of the venue, however, as there are thousands in the audience (not to worry, though, Cast Members are on hand to direct Guests to the most direct path to the park exit.) As such, you'll probably be at the bus stops by 8 p.m.
To get to Downtown Disney, you could get onto a bus for Disney's Saratoga Springs Resort, which is the closest resort to DD. Then, either hop off at the first bus stop, and wait there for the first bus to DD OR get off at the Congress Park stop; this area is directly across the lake from DD. Take a look at this
map of Saratoga Springs Resort & Spa to see what an easy walk it is from the Congress Park area to DD's Marketplace. (The pedestrian walkway at the bottom right corner of the map leads to DD, a walk that takes about 5-10 minutes.) Planet Hollywood is at the West Side of DD, though, so if you'd rather not walk that distance, then the DD bus at SSR is your best bet. Getting to a 9:30 reservation at Planet Hollywood should be no trouble at all.
Personally, if you've never seen
Fantasmic! it would be a shame to miss it that night if you don't mind a late dinner. We just love it!
Send more questions as you think of them, Abbie; we love to help!