rather than using seperate wow folders to run your characters from do a search on "Junction linking" which will allow you to run multiple copies of wow from one folder with seperate WTF folders and addons if required.

as for using the mage on follow this is quite acceptable (i've done this with between 2-4 mages on follow, behind different characters) all specced as fire (for better DPS). the only spell i found that i really needed was:

/cast [target=focus, nodead, harm] fireball

(as all followers where set to /follow, /focus and /assist party1)

one or two shots depending on the level of the mob was all that was needed to kill most things ( i had other spells setup, but only ever had to use them on fire resistant mobs) you could also bind them on the same keys as your lightning bolt on the shammy for co-ordinated attacks from both characters.