Cassie is beautiful and looks like a Yorkshire Terrier to me. It is true that yorkies and silky terriers look quite a bit alike; but also keep in mind that the standard 4-7 lb Yorkshire Terrier of today came originally from several much larger breeds. So her size & features could be from that as well as the poor breeding done in the puppy mill she came from. She may even be a yorkie/silky mix, who knows.
What ever the case, she is happy & healthy thanks to you and that is all that really matters.