Buses for the pre-opening breakfasts at the park usually start running at 6:30am, so yes, you will get there on time. When you get to the park, there will be CMs outside to check your name on a list (at least, we've always been checked on a list, some people say they haven't), and they will direct you to a line to wait for entry. CMs will let you into the park closer to your ADR time. We do 8am breakfasts and they usually begin letting us in around 7:45.
Have fun and enjoy your special meal!!!