1) Your subject selection is fine. Beware that zoology and chemistry can be tricky when it comes to css.
2) You need to prepare compulsory subjects, IR and sociology exclusively.
3) I don't know about your writing skills but still I'll recommend to adopt habit of writing essays as it's most important yet unpredictable paper.
4) There are two approaches for preparing subjects,
firstly, one can prepare 2,3 subjects at a time so that one may not get bored with one subject
secondly, one may prepare one subject at a time so that one may study deeply one subject at a time.
Final choice rests with the aspirant.