Do my arriving and departing flights need to be booked with the same airline?

I will be arriving in Guangzhou from Bangkok on Air Asia. I will be leaving Guangzhou for Toronto (with a layover in Hong Kong) on Cathay Pacific. Will I qualify for a 72-hour visa-free transit permit?

Also, if I am eligible, the application process seems unclear. When I called the airline, they said that they cannot issue any official documents, and to call the Consulate in Guangzhou. When I called the Consulate, they told me that my airline would need to request a permit for me. Who is responsible for issuing the transit permit?? Will I know beforehand, or just hope that I meet all the requirements before I board my flight to Guangzhou?