We are attempting to add a 1yr old Yorkie to our family. Pixie and my Penny are both females and about the same age/weight. Penny is pretty jealous, and the new dog, Pixie, is very stiff & nervous much of the time. She will not allow you to roll her to her side & definitely will not be on her back. My Penny tries to play with her, but Pixie ignores her.
Today they started interacting. I couldn't tell if they were playing or fighting... but it became apparent they were both being aggressive. Ears back & low, tails straight out. Pixie has growled & snapped at my Penny. She bit her & made her yelp.

Penny is constantly seeking attention from her to the point of annoying her. Is this normal adjustment stuff?
I've never had 2 dogs before. How can I tell if they are going to get along?