Hi Sushmit,
You are correct that the quizzes and mock exams use the same database of questions. On the real exam, you will be answering questions that you have not seen before. Therefore, the mock exams will have the greatest value when you take them before the quizzes. If you take quizzes first, then you will see the same questions on the mock exams, which will artificially inflate your score. I would take the mock exams first and then complete as many quizzes as you like. Note that the ITTO questions are not mock exams and could be taken at anytime.
Good luck with your exam preparations.