You can take a cab but you may be better off taking the cab to the CR, rather than to the TTC. You won't be able to take the cab directly to the MK..you must go to the TTC and get either the ferry or monorail to the MK. If you take a cab to the CR, you can just walk over to the MK...about an 8 min walk.
The buses do start running by 6:30am, to the MK.