Answer1:
Move warlock to 3rd account and create Prot Paladin on Main, that way you can alternate between Paladin and Warrior if you so wish. Or just leave as it is, and create Prot Paladin on 3rd account. Use Paladin, warlock and Holy priest for starters. Mess around with the other classes.

Answer2:
Not to sure how to have them completely minimized, but there are a couple reasons you don't want them minimized - first, when they are minimized the OS puts them in really low resource priority, and excessive lag occurs, result is that they will get lost on auto follow and so on.
Second, is that having them on the screen you can see if your toons are getting attacked by other mobs, or if they get stuck in the environment, or if they are running away in a different direction - all of this may happen from time to time and having the small screen gives you a better idea of what is going on.