The first thing I feel the need to point out is that everyone is digging up Legends, and Rebels isn't in Legends, it's in the new continuity. So Taung, the Sith race, the new continuity has yet to establish ancient history. Chances are the Republic now actually is 1,000 years old.
But, even ignoring that:
The Inquistor is not a Sith. He's specifically stated in the character previews not to be a Sith. He's a dark force adept in the service of Vader, but Vader and Palpatine are Sith of Bane's line (Bane persists into the new canon, albeit redesigned) and those Sith obey the Rule of Two.