Mate, CAAS ATPL papers only consists of 14 papers 2 years ago. They dropped Morse code, so should be 13 now. I believe that the 21 papers belongs to JAA, but I'll get back to you on it. Checking with a friend on that.
And PW4000, combined science is not a factor as long as you have Physics. I got in and I'm a combined science student too. Should also consider what Singapore flyer said as well. Those are major points of concern, especially for license currency, and if you're a CPL holder, you must do a IR test every year as well. So, do think about it and best of luck!