There shouldn't be a problem getting an early bus. Check with you hotel. we had two very early breakfasts back in August and the busses came at 6:30 and 7:00 something for early diners.
Anyway, yes there are early busses. The MK area busses start before 6:30AM. The busses for the other park areas start around 7AM.
For the Polynesian, get on the MK bus and tell the driver you need to get to the Polynesian. If he has time, he will drop you off there. If not, you would have to take the monorail from MK.