It has been purposely mentioned that he had Senju lineage and blood multiple times by BZ, Obito and I believe Kabuto. Outside of Madara only Nagato could use both Rinnegan, and Madara specifically chose Nagato as the new host since he was "the perfect host". If It could be any Uzumaki, then he had many options besides Nagato. Nagato's mother has Red hair is very likely where he gets his Uzumaki blood from while his father has black hair and black eyes and that is most likely where his Senju blood comes. Nagato seems to be like Tsunade and Nawaki and have both Senju and Uzumaki in him, making his chakra, blood and body more suited to the Rinnegan than somebody is just an Uzumaki. Thoughts would be appreciated