but celestia is more of a 'formal' and 'stern' princess and is basically a all powerful goddess. peach is more of a 'girly' type princess who loves pink and hearts and is sweet and cute and waits for her hero to come save her.
i think celestia would make a better rosalina and cadance as peach . luna is more of a 'fun' type THE FUN HAS BEEN DOUBLED princess like daisy.
(though i've been thinking, peach turns into the shadow queen and luna nightmare moon, could they possibly be a match?)