I just got a new keyboard, so it's time to break it in. :)
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Spandam mentioned during his briefing of CP9 that once they completed the "Rokushiki training" they became superhumans. Now, looking at the age of all those CP9 (well, Kumadori and Fukurou are questionable), it's pretty obvious they're fairly young adults; this would imply that none of the CP9 could teach Rokushiki to one another.
So, who taught them? It could be Lucci, but somehow I doubt he would be the teacher to Jyabura of all folks; they're rivals. I was previously more partial to thinking a former CP9 taught them, but with recent developments in the manga (that is, Coby using Soru) it could be very possible that Garp taught the CP9; heck, he might have even been the inventor of the style.
As is, there's at base minimum a week for Franky to build the Straw Hats' new ship, and things could be really bad for the Straw Hats if the Marines went to Water 7 to apprehend them while they were landlocked. So, I think Garp is going to remain on Water 7 for that week under the pretenses of "trying to capture the Straw Hat pirates", because his presence (given his reputation) would signal to the World Government that they shouldn't send more forces to capture Luffy.
Given this, as well as Luffy's interest in the Rokushiki (remember how he tried to imitate Kaku?), I think it's very possible we could see Garp teaching Luffy Rokushiki as a "training session" while Franky + Galley-La build the ship; I think this would be the perfect (interesting) way for Luffy to "up his game", since he basically tied with Lucci and he's running out of unique applications of his Rubber body; sure, Gear 3 and Gear 2 were exceptions, but aside from that there's not much room for Luffy to "expand" his repertoire of techniques, but with the Rokushiki he's opening a whole new can of worms to kick off the second half of Grand Line.
Maybe Usopp could learn the style too; anything's possible at this point.