GMAT

Hi,
1. Not all exams are not necessary, take either of GRE or GMAT and one from TOEFL or IELTS.
2. University of British Columbia - Sauder School of Business, McGill University - Desautels Faculty of Management, York University - Schulich School of Business, Western University - Ivy Business School, University of Toronto - Rotman to name a few.
3. Yes, work experience is mandatory except a couple of bottom ranked universities.

For taking admission in IIMS, there is only required of CAT exam no need of any other exam.

It depends on you. Both have equal scope, so if you want to do MBA from IIM, it is good and also if you wants to do MBA from any foreign University, it is also good, so choose which suits you.

In graduation, you need at least 50% marks for CAT or GMAT from any recognized University.

Almost, all the universities around the world (UK, Australia, Canada, etc.) accept GMAT and IELTS test score for MBA course. IELTS can be exempted, but not GMAT.

Your GMAT score is fine, means (Good > 630 > Average). Your college, class 10th and 12th score is also fine. Now, depending on your non-academic profile and job experience, I would suggest you to apply for some other good college as well. IMT (it is not specifically mentioned) doesn't accept GMAT but they do accept CAT and MAT. Anyway, IMT is good and so your profile, but apply for other good universities as well.

You don't need to appear for GRE unless you are planning for MS in the US. What matters the most is GMAT. However, as far I know there is no exemption as such in US universities for GMAT which Means you must need to score good marks in GMAT (it actually differs for every university). Additionally, you also need to have a good band score in IELTS or TOEFL (if the medium of instruction in your last attended university is not English).
Hope this helps.
Good luck.