I've read that you usually can't use Renminbi in Hong Kong.
Is it possible to use Hong Kong dollars in Guangzhou (Canton)?
Generally no. HK$ in Hong Kong. RMB in China. However, there are plenty of ATMs where your normal bank card should be able to get you cash at a reasonable rate. Some banks have "no-fee" agreements with partner banks in China. For me that's Westpac in AUS and China Construction Bank in mainland China.