I don't know if anyone has brought this up, but does anyone have any theories about how Naruto is using shadow clones when the Hachibi specifically said he couldn't use them? My guess is that instead of the Kyuubi feeding on Naruto's own chakra, Naruto is feeding it on the senjutsu nature chakra stuff he uses to go into sennin mode... Just a bad theory, any better ideas?
I have thought about it before, but I don't think that's the case. Sage mode can be spotted by looking into Naruto's eyes, and clearly he wasn't in SM when he used Mass Shadow Clones.
Yes, Hachibi said that, and from Naruto's conversation with the fox it's confirmed that the fox took Naruto's chakra and Naruto commented that maybe because of that the fox become less sinister. However, I don't think that's the reason Naruto could use MSC though.
Here are some thoughts about this matter:
1. At the beginning of the manga Kishi explained (through Jiraiya if I'm not mistaken) that the seal that Minato created is designed so that the fox can leak a small amount of its chakra to Naruto and not the other way around. However, the conversation with the fox made that explanation contradicted each other.
2. When Naruto took some of the fox's chakra, Naruto performed a sealing technique that we didn't know how and/or when he learnt it (The chapter titled "A New Seal!"). The fox said, "It's the sage of sixth path's ...", what exactly the fox meant by that? Is it SOSP sealing technique or is it something else?
3. If Naruto could use SM and the 9 tails mode at the same time, it doesn't make sense for him to feed the fox with the nature's chakra, risking turning the fox into a frog rock.
4. There are only 2 logical explanations about the MSC and 9 tails mode, 1st, the fox and Naruto has reach an agreement and 2nd, somehow Naruto can seal his chakra from leaking to the fox. From the 2, I think the 2nd would be the most logical.