Hmm... I think that I may have figured out the reason for having Random Blond (additional?) Gein as one of the opponents in what I'm fairly sure is Kanryuu's mansion. If the RK movie is going from "You are carrying a sword! Clearly you must be Battousai!" "Sessha wa... okay, yes, fine" to Sano to Jineh to Megumi to Gatling guns to Saitou... yeah, you don't really have time to include "And now we will spend twenty very talky minutes on each Oniwabanshu member, with extra time for Aoshi to explain that he has mad kodachi skills."
On the other hand, you do need fight scenes leading up to the Gatling gun, and you need somebody to be able to kidnap Megumi. Ergo, Gein. And apparently Camo Guy from the Jinchuu Arc as well.
(on a not-unrelated note, I shall be VERY CRANKY if they go the route the manga went and have Kenshin and Sano take Yahiko with them to rescue Megumi, while Kaoru stays behind to heat up the furo and make a complete muck of dinner. Seriouly; the kid has had maybe a month, tops, of kendo instruction at that point; it's the worst example in the entire manga of Watsuki-sensei utterly screwing up the characterization in order to have his author-expy character get to go do cool stuff. The anime fixed it, so hopefully the movie will go with the version that actually makes sense. /soapbox)