so can we assume the juubi was made-up by madara aswell? or is there any proof to back up the existance of the juubi i'm forgeting
Well i won't go as far as to say the juubi is a lie, becase the SO6P sems to have really existed. Because when naruto took the foxes chakra he took on a firery figure of the sage's silhouette.
It is at this moment he got a new seal to seal the fox with out of no where. So this suggest if the SO6P really had existed he posses a special ability to seal, which he could use to seal the Juubi. Which he is said to have done, so yeh I really think the sage existed.
But even though the sage may be real you can't totally take madara's word for everything, he maybe either twisted the story a little or left out major parts of it. Madara lie alot especially to sasuke .....!!