Computer no. is only issued once so you can use it for any of the sessions, and the same no. is used for your atpl exams and any other technical exams that you might sit for in the future.
No separate statement for ground school is needed for application of the computer no., your canadian PPL and all the documents asked for in the application should suffice. You don't need to give your spl no. as you already have got a PPL. When I applied I gave only the documents that were required in the application form, no extras.
http://dgca.nic.in/forms/Pilot/compno_pilot.pdf
http://dgca.nic.in/forms/Pilot/pilotexam.pdf
I did the CPLCG exams and only needed the annexure1 signed by the CFI as regards to ground school, no additional statement for ground school is needed for that either. I don't know what the new rules are regarding eligibility for the exams, as they keep changing, but getting the computer no. with a foreign ppl should be easy.