Hi, I think you have to stay over in Guangzhou since the buses from downtown Guangzhou to Taishan will be stop then. Well, when you arrive at Guangzhou Airport, please use the airport shuttle bus Line 1 to reach Guangzhou Railway Station. Here, you could find a hotel for one night. On the next day, please go to the Provincial Bus Station of Guangzhou, which is just located to the west of the railway station. You could take a bus to Taishan from 06:20 until to 20:55 every 30 minutes. The journey is about 2.5 hours. The bus fare is CNY60.