Ok, making up a class for Rarity is both silly, unimaginative, and failing to recognize many aspects of her character. Off the top of my head, an Illusion-focused Sorcerer would probably be suitable, for the idea of being able to create beautiful things as an innate aspect of her very being would appeal to her. An Artificer that focuses on the elegance and beauty of her creations just as much as their functionality might appeal, too. (Bard would also suit her for similar reasons, but Pinkie always calls the role).
Also, Twilight being a wizard seems... off, too. I mean, being an *actual* wizard is her *day job*. Who wants to play and RPG to play their actual job, however gamified and abstracted it is? I'd peg her as a Cleric- why? well, she's absolutely one to jump in a needed role, and every group needs a Cleric. Second, she's also the type of person to read the entire rulebook cover to cover and recognize how powerful Clerics can be- and third, she seems like someone who would absolutely love playing a character who worships a sun goddess. :3