I'm with the others who have always had a successful bus trip for early character breakfasts. sometimes the busses will run as early as 6:45, but that's mainly for those with very early reservations at Chef Mickey's.
If you're at the bus stop by 7, you should be just fine, unless something *really* goes wrong with the system.
From my experience, the MK gates will not open until 8. At that point, everyone with 8:05, 8:10, 8:20, 8:30 reservations all head straight to their restaurant, so the first half hour or so of seating can be a little chaotic.
(This is also how one might get seated early - just go to the restaurant at opening, and they normally don't worry a whole lot about the exact time of your PS.)
Taking a cab might actually delay you, since you'll have to transfer via the TTC. If you decide to do that, my suggestion would be to walk over to the Poly and catch the resort monorail to MK. It's about a ten minute walk from TTC, and very pleasant if the weather is nice.