To answer question number one: brushing is important as soon as little one has a tooth. As soon as little one has several teeth next to one another, it is important to brush AND floss. Crazy silly to put in practice...but necessary and true. Twice a day!
Fluoride in *excess* is poisonous (even the natural/environmental factors can make drinking water unsafe - like in areas of China). Fluoride in the correct amount is desirable. It arrests oral disease and deterioration. Although it isn't related, I *think* about it sort of like iron. Too much = not good. The right amount = necessary. To little = not good.
It's tricky though, because toddlers will often swallow their toothpaste. This, in addition to fluoride added to city drinking water and/or other fluoride additions, can amount to TOO much fluoride. So, INHO, it is best to talk with a *trusted* dentist to figure out what is best for your child, their age and in for your area. (I'm in an area that doesn't fluoridate water, so it's actually not harmful for them to use fluoridate toothpaste, even if they're swallowing trace amounts.)
Also, depending on your diet and the amount you brush/floss, your need for fluoride drops or increases. My friend, who is a dentist, does not use fluoride toothpaste because she has a good diet, brushes and flosses multiple times a day and does a fluoride rinse (not sure if that is proper terminology) once every few months.
I love the world health organization and have found their articles very interesting:
http://www.who.int/oral_health/events/Global_consultation...
http://www.who.int/water_sanitation_health/oralhealth/en/...