For the most part, they will. They are NPCs at the end of the day, so there may be situations in which other players can handle them. One magical "field" per mage, correct. Their magic is inherent, dictated by genetics. A Fire Mage will always be a Fire Mage, for example.

Also, I've got the [url=http://www.roleplayerguild.com/posts/2972442]OOC[/url] up now, so you can post your character there when you're finished.