@Nphl,
CAAS requirements:
3.13 What qualifications will I need to apply for the initial issue of a Singapore Commercial Pilot Licence (CPL) and Airline Transport Pilot Licence (ATPL)?
i) You must not be less than 18 years old (CPL) and 21 years old (ATPL)
ii) You must pass at least five subjects at the GCE 'O' Level including English Language, Mathematics and a Science subject.
iii) Pass a Singapore Class 1 initial medical with a CAAS DME
The short answer to yr first question: Yes, you can apply again but you will be rejected.
As for your second question, it is a straight No. It does not give you an advantage over others. All applicants are treated the same. If I could give a small piece of advice, just study something that you are really interested in. Do not waste time and effort putting yourself through a course just because someone told you it will give you a headstart. I'm not trying to be a wet blanket but the chances of an applicant getting through to the cadet pilot stage is less than 10%. You don't want to find yourself regretting the decision to study something ya never liked.