Well, considering Yumi's souer issue. Even if she WAS to pick random-run-in-girl-X right now, wouldn't that be quite a kick in the shorts for Touko? Despite what she said, judging from what you novel people have hinted, she seems rather eager to be around Yumi.
I know EXACTLY how Yoshino's souer story will be!
1. Mysterious super-kendo genius girst year joins the kendo club.
2. Gets friendly with Yoshino
3. Rei gets jealous and the both of them initiate a kendo duel.
4. Rei kicks her ass, but in an even match.
5. Rei gets her ass flamed by Yoshino for being overprotective again. Yoshino drags the girl off, making her her souer out of rebellion and as a reminder for Rei if she ever gets the mother-chicken syndroma again.
*gets thrown out*
.......well, this is how I imagine it anyway ^^;;.