If you don't know anything for sure, don't assume anything. Just train her like any other Yorkie, with some extra sensitvity.
I believe it is most certainly possible to get her housetrained, so just keep working on it, and don't give up hope. Remember that things will get much better as she adjusts to her new home and begins to trust you. Poor baby has been moved around alot, so nothing in life probably seems very secure for her right now.
Also, could I ask what rescue you got her from? I'm always interested in rescues in CA I can refer people to.