Some lighting questions

Hello All

I have been given some advise on lights for my grow but I have a few lighting questions.

1. with HID must the bulb and the ballast wattage match or can one be higher then the other?

2. What is the difference between a "switchable wattage" ballast and a "dimmable" ballast? (And why have I been told to avoid the dimmable?)

3. I will be using MH and HPS so I would want a ballast thats switchable between them. Some ballasts are switchable with a switch and some auto detect is this correct? And they can be Magnetic or digital? And if magnetic can it use CMH?

4. Whats a conversion bulb?