Yeah, Shuri was mentioned in NT to look like a child due to some sort of disease.
It doesn't surprise me that there are more ninja hidden in AC, especially given Shuri's group's goals of acquiring supernatural powers. I really like the Scavengers, so them being recurring characters in the Railgun manga is awesome. Of course, Leader's plan is sure to blow up in their faces, just like the last 2 times they encountered a Level 5.