It is never stated that the Uzumaki descended form the Senju! God, I hate when people say that. The Manga states they are "DISTANT RELATIVES". Nowhere does it state that one descended from the others. But seriously more than anything, all the facts/tidbits point toward the Sage being an Uzumaki.
If he had the Rinnegan already why didn't he just control the Juubi instead of sealing it? Answer: He didn't have the Rinnegan until he sealed the Juubi and it later manifested while he was the Jinchuriki.
He survived the extraction of the Juubi (In which he later sealed in the moon). Who are known for their strong life force and known to be able to survive the extraction of a Bjiuu? KUSHINA (An Uzumaki).
What are the Sages tools specialized in? SEALING. What did the Uzumaki clan specialize in? SEALING.
When people say "The sage was both Senju and Uchiha therefore it debunks this theory", that is also wrong. HIS OFFSPRING CAME FORTH WITH THE LINEAGE OF BOTH CLANS. The clans are related to the sage but their bloodline directly stems from the sons, not the Sage himself (Even with some relation). He is technically the Grandfather of both clans.
I could go on with this....