Hopefully I can clear up a few things.
I got Hiraishin from Bocchiere, and as for the '1 for 1' rule, that was never established. Aaron has always had more than one student, or he does now, at least. Or did. Aaron's scroll makes it so the knowledge cannot be taught, stolen, or anything like that.
Bocchiere learned from Tommi years ago. When that happened, he was able to teach whoever he wanted. During that time, he didn't make it so people who learned from him couldn't teach, or he did so right after teaching me, because I clarified I was going to make my own scroll and teach others.
That being said, I DO follow the only one student rule and have a scroll that works like Aaron. No one can teach it or have it stolen from their heads when signing my scroll and learning it. I have taught Keito and my other alt, Hazama, was taught it by Bocchiere as well and I don't intend to teach it to anyone from him.
As for Tokujiro, Masane, and Ichirou, that happened because of Tsuyo. When Tsuyo learned everything Bocch did, he gained the ability to teach people. Tokujiro was taught directly by Tsuyo, I am pretty sure, and Masane and Ichirou learned from... I don't recall, but from Tsuyo's stuff or his training, one of the two.
As for everyone else, well, I haven't the slightest. I hope this helps a bit.