Certainly no problem using credit cards anywhere in Hong Kong assuming they are Visa or Mastercard. Like everywhere in the world the number of places accepting others like Amex or Diners Club is a bit lower.

In major Chinese cities you won't have any problem using credit cards at hotels and major shops / restaurants, but as the previous poster said it would be wise to have enough cash with you just in case.

You really don't need a driver in Hong Kong - public transport is superb and cheap (get an Octopus card). Taxis are also very plentiful and cheap, and if you are just touring around major tourist destinations then you will have no problem with language (and if you do then they just get someone on the radio who can translate for you).

In the mainland I guess you are staying at major hotel chains - in that case I would just take the hotel's recommendation on a car service. It will probably be ten times the price you could get a local for directly and in the absence of personal recommendation you will probably be happy to pay that for peace of mind.