That is fairly easy considering the MK fireworks aren't that long. My advice would be to watch them from the end of Main Street near the entrance of the park (right before going under the train tracks to exit). This way you can be one of the first people out after the show. Then run on to the Monorail to the TTC and transfer to the Epcot monorail. Once in to the park go towards World Showcase since Future World will be closed.
It seems like this will be east to do as long as you move quickly. If you really want to be on the safe side, watch the MK fireworks from outside the gate of the park. This way you are one of the first on the monorail.
If you want to try something interesting, you can leave the MK at 6:30-6:45 and watch the MK fireworks from the Polynesian beach. Then you can walk over to the TTC and get on the Epcot monorail.
Whatever you do you should be safe as long as you don't waste time. Have Fun!!