Where do you think the inquisitors came from? The one in Rebels hinted at this fact.Also, just a random thought, but what about all the "failed younglings" that joined the Medcorps, Agrocorps, etc? We've learned from other EU sources that even those weak in the force can eventually become proficient and strong in the force, given time and inclination. We've never heard how they were affected by Order 66. It's theoretically possible that a number of them survived.
Nice, I didn't know that. I haven't watched Rebels yet, but that's a pretty cool idea.