One can depend on Earthdawn sources, to a point. (Namely, the point that the two properties changed companies.) Regardless, pixies, along with other sapient critters, are indeed races that existed in previous ages of magic, but they needed more magic to return to the world than, say, elves and dwarves. As for the residents of the plane of Faerie, there are two possibilities. One, some pixies got trapped there during the downtime after travelling bodily to the metaplane. Two, those aren't actually pixies, but spirits taking that or a similar shape. In my opinion, the second is more likely, as we know that (meta)human thought and myth affects the metaplanes, and vice versa.