Personally, I wouldn't worry about transportation at OKW. Here's a few things to keep in mind:
Old Key West is a very laid back, quiet, and relaxing resort. As such, time passes very slowly. Buses run on average about every 15 minutes, however, that might feel like longer.
Old Key West is also larger than it seems. There are 5 stops within the resort, and lots of turns. It typically takes me 10 minutes to go through the resort, making all 5 stops. Depending on where you are, you might not have the full 5 stops, you might have less. Theme parks typically follow the following order:
Peninsular
South Pointe
Turtle Pond
Millers Road
Hospitality House
That's how most of the buses will go. However, during the park exit, many drivers will stop at the Hospitality House first. Also, myself, and many other drivers will change around the route, if the only guests on our bus are going to one of the last stops. Not always possible, but if it is, we try to accomodate.
Old Key West buses are seldom crowded. From 10am until park close, buses typically run with 0-10 people on board. Even during peak times, standing is a rarity. (It can happen, though)
I overheard a driver talking to another today. Saying how busy things have been. "Yeah, it's like Summer again. I was doing the OKW this morning, and I must have had 25 people on EVERY trip!!" (Yes, 25 people is a lot...)
If you give us the chance, I'm sure the transportation at Old Key West will be just fine. However, taxi's are readily available at Epcot and the Studios, just past the OKW load zone. AK is off by the charter bus lot. And the taxi's at MK are at the TTC, which means you'll need to take the Monorail or Ferryboat.