1. Why dimensions are in millimeters and elevations in meters (generally)?
Accepted convention is OK but there would be some precedent to it.

2. Underground minimum connection size is 2″ in most of the cases. What is the reason?
Kindly correct me if this is not the thumb rule.

3. Slope is 1:500 unless otherwise stated for sloped lines. Where does it have its origins?

Regards,