i used to take the T from long ave in allston at 5:15am, i dont know if there's an early one. however police seem to not like it when a guy wearing all black with a hood over his and a packback on is running and jumping off hoods of cars to catch said T, at least not that early in the morning they didnt.
but you could just go to the mbta website and check out the start times
a bus, why not just take the greenline to the redline and then catch the silver line i mean that is so much easier then a bus. or wait is it, hmm tell us when you get back.