First off been here a while and just stumbled across this... (noob!).
Anyho I will assume that if I have a Heal Priest, Warlock, Hunter, Rogue I could download different UI packages and just make sure I change the proper file names that are reflective of each UI/class.
Along with that I would need to "untic" mods not used on each character at the Character selection screen.
Are these safe assumptions?