If you want to toy with Puella Magi using their Witch forms, Kyouko could qualify as Rider. Her Witch, Ophelia, rides a pesudo-mechanical unicorn-thing.
Nagisa manifesting Charlotte separately and riding her snake monster form would work as well.
I could see it working both ways, with some classes having the Witch and Puella Magi separate and only using one or the other, and others having them fused (like with the Law of Cycles girls in Rebellion and Sayaka in this story). But if they were to be separate servants, I think the Witches would only qualify as Berserkers. Their madness is too deeply entrenched in their "legends" to be anything else.