Wecome to yt!!
The potty training issue is one that we all know!!! It is NOT impossible to train them.... but you have to be diligent!!
You may want to use an xpen to help keep the puppy out of the litterbox if you are busy and can't keep your eye on her. Or maybe a gate that the cat can jump over and Pixie can't.
Check out the threads on Potty Training. Just do a search on them..... there are MANY, and lots of great advice!
Good Luck to you and your new Yorkie!