I think the interpretation of the Mazou being the Juubis shell (at least if you take it literally) is utterly wrong. It is something created by the so6p as well.Because the Juubi has a shell that makes up its body, the Mazou. All the tailed beasts as we know them don't have an actual shell, they are essentially just masses of chakra. So its more logical for multiple individual tailed beasts to exist for this reason. But for the Juubi to multiply the entire size of its body, it would logically have to split the Mazou down to a smaller form as well.
It does however not change what I said before. All you can do is nod your head, and consent to what you read. It does not have to be logical to you, all that matters is how Kishi sees it. Also you cannot "trust" whatever information given by characters in the actual manga. Madara might not know everything. We have seen this on several occasions, where one explanation is either totally wrong or becomes modified along the way.
I personally believe, that the so6p could meet Naruto, and he might give the 'most correct' explanation about everything regarding the Juubi and his battle with it. There is definitely a reason for the so6p to create the tailed beasts and tell them, what we saw in Kuramas flashback so far.