Think about this: without KabutOro, Madara's war would have been crap. Think about it, they have the hardest time with Edo Tensei, not with the weak ass Zetsu clones. Releasing the Bijuu would be an option, since he can control them, but still, 7 at the same time might be overkill, even for him.

Maybe the manga adresses the Kyuubi chakra thing, but this it the (weak ass) excuse I came up with:
The Raikage mentioned Gin/Kin were in its stomach for 2 weeks, thus becomming infused with it's chakra. Maybe they were able to (to an extent) generate it themselves. Otherwise they might've spent it all while they were alive. They do somewhat make Edo Tensei plot-hole proof by letting KabotOro mention that nobody fully understands the extent of the jutsu.

The same weird stuff is going on with Naruto himself now. It is said that they only managed to seal half of Kyuubi's chakra into Naruto, because it was too much otherwise.. but wouldn't Kyuubi recover the chakra eventually?