Re: A SO6P theory....
Nice theory, but you need more research to back it up! However I've always thought something similar. We know Naruto is HEAVILY influenced by Buddhism and Chinese / Japanese history. The SO6P story / powers are strikingly similar to many buddhist beliefs. (6 paths, Kannon etc)
Often times in the manga he was referred to as a Monk. I believe the Uzumaki clan were monks. Those who practiced Buddhism. The SO6P must have obtained enlightenment or something and discovered Chakra. Thus being the forefather of Ninjutsu we know today. It is also possible, and highly likely he mastered Senjutsu. Thus gaining the title Sage.
To elaborate on your theory, I believe he mastered both inner chakra and external chakra. (inner path, outer path in buddhism?) Created Ninjutsu/Senjutsu, used this to seal the Juubi. Thus gaining the rinnegan. (like how Naruto has lines on his face from Kurama).
If the Uzumaki clan is where both Senju and Uchiha DNA originated, that would PERFECTLY explain how/why Nagato was able to receive and control the rinnegan. Maybe this whole time Madara was trying to make himself an Uzumaki? :D