All the Sages of 6 Paths
According to many people on the base, there are 6 people who denote one of each Path of Pain. In my opinion this is false. Most people get it from what Obito said when he was talking to Nagato's body: http://www.narutobase.net/manga/Naruto/510/16
As we all know, the So6P is the one who first awakened the Rinnengan as seen here: http://naruto.wikia.com/wiki/Sage_of_the_Six_Paths
As to go along with my theory, since he was the first person to awaken the Rinnengan, he will be known as the First So6P.
The second So6P is none other than the second person who awakened the eyes. In other words, Madara: http://www.mangareader.net/naruto/560/15
As seen at the bottom of the page, Kabuto says that Madara has acquired the powers of the Sage of six paths. But how do I know he is the second So6P? Well this is based on what Obito said about Nagato: http://www.narutobase.net/manga/Naruto/510/16
As we know, Nagato is the 3rd person to awaken the Rinnengan (although he didn't actually awaken them, he used them and is therefore considered a person to have the Rinnengan in his bearings) and Obito denotes him the third So6P. By this same token, it is for this reason that Madara must be the 2nd So6P, since the person who awakened the eyes after him, was known as the 3rd S06P.
By this same logic, we can see that Obito is the 4th So6P when he uses the rinnengan: http://www.mangareader.net/naruto/564/10
Naruto states "just like Nagato and Madara" in this page and since he is the 4th person to use the power, he is therefore considered to be the 4th So6P.
IMO, no manga proof or anything, but sasuke will become the 5th So6P and Naruto will become the 6th So6P. why do I say this?
Well, Sasuke will lose to Naruto in their final battle (if he decides to be evil) and since he needs a HUGE powerup in order to even fight against Naruto, I think the Rinnengan will be a good addition to his arsenal of eye techniques. Eventually he will lose blah blah blah and Naruto will reign on top! Then, as we all know, he will become the 6th Hokage! See the connection? The number 6. IMO, it is this connection, the fact that he is going to be the 6th Hokage, that will also play the fact that he will become the 6th So6P.
Although this whole part is speculation, the other part about Madara, Nagato and Obito being Sages because of their usage of Rinnengan, ultimately means that people who acquire the rinnengan, are known to be Sages of the 6 Paths.
That is my theory guys. Hope you liked it! :D