Well, I don't know for sure if they only transform with their torsos... I can't recall seeing anyone glomp their legs or anything, so who knows if that would make them transform. But it does seem that even when Kyo or Yuki stand close to Tohru, they make sure not to touch torsos together.
Spoiler concerning REN wrote:
Yeah, I did mean Ren. Akito's mom. Not Rin/Isuzu.
And no, I haven't read the
entire series, just all of the English, a bit of the French since there are more French volumes released, and a bunch of major spoiler summaries/scans on various Fruits Basket sites for up to volume 17-18.