can i get a coach from changsha to guangzhou?

Yes, of course. From CS East and South Bus Station, you can find direct buses for Guangzhou.
From the East Bus Station, the buses are at 16:30, 17:30.
From the South Bus Station, the buses are at 11:00, 14:30, 17:30, 18:30, 19L00.
The bus fare is around 200 to 220RMB per adult.