In Sept., four of us will travel from the US arriving at Hong Kong International Airport at 7:10pm.

According to your arrival time, there is no direct bus that can take you Zhongshan. Thus you are advised to collect the luggage as soon as possible. Then go to Skypier and catch the direct ferry to the destination. The last ferry schedule is 20:30. The trip takes around 70 mins. And it costs about 250 HKD per person.