Also, you didn't seem to address the fact that Yoda himself must have been a Dark Lord of the Sith in order to have had knowledge of the rule, as it was put into place AFTER they were wiped out... and that comes from George himself.
If Yoda is meant to be one of strongest Jedi of all time, wouldn't it stand to reason that at some point in his 900 year life, a Sith lord tried to 'convert' Yoda to the darkside, perhaps even Darth Sidious's former master, assuming it's an unbroken lineage of two at any one time.
It would explain Yoda's knowledge of the Sith.