The criterion was “What faction might they join if they popped up in Sigil, had to stay there for a while, and realized you should probably sign up with a faction if you’re going to be getting things done around there?”
Yes, other matchings could be made. Pinkie could totally be a Sensate. Or a Cipher. Applejack could certainly be a Godsman, putting in her hours in the Foundry. Et cetera.
Hm. She took lessons in assertiveness, and learned the lesson well enough to integrate it to the amount she desired. She was afraid of singing in public, but overcame her fear. Being shy doesn’t mean you’re not trying to improve yourself.
But hey, who do you think she’d be better with? I was trying to think of a faction that makes any point of caring for animals and didn’t really come up with one. (The Wilders are a Beastlands sect.) The Godsmen at least think you could have been one in a past life.