If i modified one character, adding an animation, would others have that animation too? or would it be a custom animation that only that character can perform?
That depends on WHERE you put your animation.
If you integrate it into supermodel it will be useable by all characters, who use this supermodel.
If you add it to one of the armor models - it will be useable by all characters, who use this armor model (but only when armor is equipped).
If animation is stored in specific character model (like Malak, Nihilus or custom made models like Sith Stalker
) it's useable by this characters only.