from Guangzhou airport to J Hotel at Tianshou Road, Tianhe District

Yes, you can get a taxi to J Hotel at that time. You have no need to book it in advance. No surcharge for taking taxi during mid night in the city. It charges CNY10 for the first 2.5 kilometers (1.6 miles), and CNY2.6 for succeeding kilometers. You can bargain with the driver if you take a taxi at night. J hotel is within walking distance to the East Train Station. The taxi fare at night may be around CNY150 from the airport to the hotel. A cheap way for you is to take the airport shuttle bus line 6 to CITIC Plaza, close to the East Train Station and then walk about 500m to the hotel. It only costs CNY20 per person.
Currently, the weather is mainly sunny. The temperature is about 20-30C. Light clothing is OK. You can just prepare a light jacket for night time.