AC questions for the experts

R-12 and R-134A are "low temp" refrigerants. R-22 is a "medium temp" refrigerant. You have to understand that a refrigeration system is designed to operate around the characheristics of a given refrigerant, boiling point specifically.

As far as the test required for the purchase of refrigerants to help satisfy the Montreal Protocol, anyone can likely pass it if you have a rudimentary knowledge of refrigeration and air conditioning. The test involves your knowledge on container handling, legal issues with recovery and disposal of the refrigerants, as well as storage and recordkeeping. They tell you everything you need to know just before you take the test, and it's not really about working on anything other than prevention of loss. There are multiple sections to the test so that people can pass and be legal for their given field, yet maybe not be legal for stuff they dont mess with, i.e. Ammonia.