Hi, Maria:
It is Ok to go back to Hongkong at the same night.You can take a train to Luohu, shenzhen and go back to Hongkong, the luohu boarder is working until 12pm. Or you can take a direct train from guangzhou to Hongkong, the last train is 21:30. So i think it is enough time.

If you take the bus, you will pass the Huanggang Pass, and it is open for 24 hours. While if you take the train from Guangzhou to Hongkong, then you will pass the LuohuPass, it will be closed before 12:00 p.m. during the night time. Allin all, time is enough for you to clear the customs either you take bus or the express train.