. . . . and if you take a cab, it might be best to take to either the Contemporary and then walk, or to the Polynesian or Floridian and take the resort monorail.
Catching the express monorail at the TTC that early may be problematic from day to day.
I would try to get to the bus stop between 6:45 and 7. I know they say to allow at least 45 minutes to get there. Sometimes it takes less, sometimes it takes more.
And getting there a few minutes early will give you guys a chance to take a few pictures on Main St. and in front of the castle without a big crowd.