Pretty good thread with one disagreement; The Sage was a monk who wasn't a ninja himself, but passed on teachings in form of Ninjutsu. Presumably, these were basic Chakra manipulation principles to harness the spiritual essence of individuals into techniques, which ninjas perfected later into a proper discipline or facet of Ninja abilities. That leads me to believe that he only developed that special Ninjustu - the sealing ninjutsu - for the 10 tails. He couldn't have written that Tablet then, as the manga doesn't offer any insight on this issue that he even knew any other sealing techniques beyond this, as the man after all, was not a Ninja, but a monk/sage.
Great theory though.