I think he's got to find a way to access their grave first, though now I don't remember why I have this impression. *scratches chin* it's possible Shisui was cremated or something. Or else he'd Edo Tensei Rikudo Dennin himself. Then again that might be who's in the last box that scares Madara so much XDDD

Kabuto did say that he had perfected/leveled up Edo tensei higher than Oro was able to, then again oro wanted to know all jutsu, I think Kabuto would be able to pause and devote more attention to one of them only where orochimaru didn't necessarily take that time since he had still more jutsu to learn. Also it seems to demand a lot of chakra... now why the hell kabuchimaru has more than orochimaru had, I got no clue. Maybe the reincarnation trick, or maybe the fusion is getting Kabuto's cells all excited, who knows.