If I'm preparing for the CAT and XAT together, do I need to study anything extra for the NMAT IIFT and SNAP?

It is possible but not advisable, as you may not score as desired. It is important to cover the SNAP syllabus too. You can solve questions in mock test correctly only if you understand the topics well. 

• For every correct option 1 mark is awarded
• Forr every incorrect option 0.25 marks are deducted
• No deduction for unanswered questions

SNAP exam is a computer based test mode. Candidates have to visit the allotted test centre and take the test on a computer system. 

Yes, six months are enough to prepare for SNAP exam. Candidates should start preparation by June to be well prepared by December, the exam month.

SNAP has negative marking of 0.25. To avoid negative marking, candidates must not do guess work and do not attempt any question that they feel they can't solve correctly.