The end to all of your theories, the VIZ scan:
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He possesses the blood and powers of both the Senju and Uchiha clan; the clans were once one and the Sage was the originator of that clan
He is
NOT an Uzumaki
Uhhh...
All it says is they descended from his line, in no way does it say he is Senju/Uchiha because they didn't even exist. Have you even done any real research like those of us who contend that Rikudo Sennin was indeed an Uzumaki/Hyuga hybrid? No, of course not. You just want your favorite character to be important by association, well too bad, the manga is called Naruto. He is an Uzumaki. Thus it would be logical to assume that Uzumaki's are far more important than Senju or Uchiha both of which are virtually wiped out (the only Senju we know for sure that is living is Tsunade). He had great sealing techs, had to have great ocular acuity which was then mutated by having the juubi sealed within him, and chakra strong enough to invent ninjutsu and create the nine bijuu. No Senju or Uchiha has those sealing capabilities but the Uzumaki do. The Senju and Uchiha don't have perfect ocular perception but the Hyuga do, thus some of us conclude that Rikudo Sennin was a combination of Uzumaki/Hyuga.
Your argument is wack to say the least(and yes I'm trying to bring that word back). People click the links below.
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Edit: Also, look at the symbol in the middle of the ripples and nine tomoes, It's the symbol for the Hyuga clan, could it be that he had a third child? Doubt it. Could it be that Rikudo Sennin has Hyuga lineage? It's possible.