I think all my characters are fine... The Cures are obviously easy targets, being used to attacks that deal straight damage and not convenient energy-drain effects, and Totori won't really be able to do anything about them - then again, if they'd tell her their purpose, she'd probably let them take as much as they wanted and then start work on the strongest curative she knows at the time. Hayate will get whatever Cure stuff is applicable from Itsuki and Kanade, and will get item crafting magic from Totori.

Rita, on the other hand, creates a weird edge case. She herself is fine, and her party doesn't have magic, with one big exception. Estelle's magical condition is something that drives plot in the last act of the game, and I somehow think that draining her magic will have a severe negative effect on canon. So, uh, if we work out the Wolkenritter coming to Terca Lumireis, Estelle will need to be either kidnapped or otherwise absent at the time, because if they drain her we're getting a nonstandard game over (and the Book will probably be done). Hayate would learn Rita's artes as of when the Wolkenritter come calling, which could possibly be up to her early expert spells... I don't envy having to deal with Violent Pain. And heaven forbid if she somehow learns Indignation, that thing hurts.

That said, Hayate's Knights (and Hayate herself, in peacetime) are welcome in any of my characters' worlds, barring an objection from my castmates, for good intent or ill.