HK Airport to Casa Hotel, Nathan Road after mid-night

I would be traveling in a group of 5 (two adults and three children below 12 years of age) arriving at HK airport almost at mid night this coming November 2012.

Are the taxis in HK allowed to take in 5 passengers or would I need to split my group up into two taxis?

Also, do taxis in HK charge for double after mid-night? What would be the estimate cost be for taxi transportation from the airport to the above said hotel after mid-night?

Would it be cheaper for my group to be taking the airport express followed by MRT to the hotel? (I noticed that Airport express and MRT services services are available until 10 minutes to 1am). Thank you.