After reading your post, your theory is a possibility.When A(Raikage) said that the Gold and Silver brother could have been descendents of the Sage of the Six Paths because they live after eating the nine tails flesh, one of my earlier theories that there had to be more clans than just the Senju and Uchiha who were descendents of the two sons was somewhat confirmed.Now who was first, that's a tricky part.The only thing we have to go by is what Tobi said, it seems like the Uchiha and the Senju were the first two.Now Tobi said through the ages the descendents of the older son became known as Uchiha and the descendents of the younger son became known as Senju.We have no way of knowing if any of the descendents of either brother started their own clan before the creation of the Uchiha Clan or Senju Clan.Just going off what little information we have, I would have to say the Uchiha and Senju Clans might have been the first two.