Their is no evidence. :| We case dismiss "that" case. We know that Sharingan evolves into the Rinngean. However, we do not know if the Sage of Six Paths is the "true" Rinnegan wielder (i.e if he was born with Rinnegan or was Rinnegan a mutation).
Yes he was born with the Rinnegan. It's safe to assume this since we've seen that being the Juubi Jinchuuriki doesn't get you Rinnegan. If it did, Obito would have had both eyes Rinnegan.
As for Hagoromo needing / having Susano'o I don't think so. Not because he couldn't have it, but because he just wouldn't need it. It's safe to assume that the Ninjutsu / Rinnegan Techs Rikudou Sennin used were far beyond what we know already. Omnyoubokun (whatever it's spelt) is one fine example. I think it's fair to assume that he had something more powerful than Susano'o.
And yes, it's also possible that the Rinnegan of Hagoromo shared Sharingan techs, since basically Sharingan is a weaker version of the Rinnegan with different default powers. while the Rinnegan could get the Sharingan powers, the Sharigan can't get the Rinnegan's powers, that's just common sense / knowledge.
If you ask me, I don't think he used / had 90% of the jutsu's we've been shown in the series, but something far greater than any jutsu. He was Rikudou Sennin after all, a God.
@ KCM Naruto: I love your post and those pictures. Fellow Rikudou Sennin fan, respect!