Taxis in Beijing airport, do we need to hail for them or they are already in queue.

No, as I know, the cabs have already in queue. There are still many cabs at around 6am. Don't worry.
By the way, there are several pick-up stations in terminal buildings:
T1: Outside Gate 1 on F1
T2: Outside Gate 5 to 9 on F1
T3: Please refer to the signs inside the terminal building
Hope helpful!