You can cut Jillie's hair, just not as short as Piper's. My Momo has soft cottony hair & I like using the #7FC blade on him & the other 3 who all has silky hair gets cut w/the #4FC or #5Fc (in the summer) blades. For blades, the higher the #, the shorter the cut. If the silky ones gets cut w/the #7FC blade, they will look like they have very short hair, almost look shaved. I like leaving a little bit of hair on their body, it protects them from the sun. It can get pretty hot & humid here in CA & I do grow their hair in the cooler months to keep them warm.
Here are some good links to different Yorkie hair styles & cuts.
Yorkie Hairstyles and Hair Cuts - Pictures Explore Yorkie Haircuts Pictures And Select The Best Style For Your Pet