Can I carry a samsung cell phone (not galaxy 7) into and use in china?

You are allowed to take it into China. As I know, unlocked cell phone can be used normally in the country.
There are three mobile operators, China Mobile, China Unicom, China Telecom. If you want to buy a sim card in the country, you just need to figure out which operator your cell phone can support and buy the card at the business hall.