Re: A SO6P theory....
In my opinion, these two scans should already be enough proof that Rikudo Sennin was from the Uzumaki clan.
Notice how Kurama says this is the sage of 6 paths, right after Naruto SEALS him.
Plus, what did Rikudo Sennin do? He sealed the Juubi.
Also, notice that Kushina, for example, was the only one to survive Kyuubi extraction + survive as well, for a little time, after she was impaled by Kurama and still had chakra = strong body/chakra pool. (Senju)
We know that she had a special chakra which was very powerful to tame the Kyuubi= powerful chakra (Uchiha)
Clearly in the Uzumaki traits we find both of the Senju and Uchiha (younger and older son) traits so it's not wrong to presume that the Sage was Uzumaki.