Is my dog being 'dog aggressive' or no?

I have a female Shih Tzu that I adopted from an abusive background. She is really bonded to me. She has only snapped a few times, once when I accidentally picked her up wrong and it hurt her, and when she was scared because of another dog and I picked her up from behind. She has no bite history, and has never bitten, just snapped like she was really irritated and not directly at me or anyone else like she intended to bite. I have only had her a few months and she is the sweetest dog with me and once she gets used to people. She does good around other dogs, ONCE she has been introduced. If they get near me then she will growl at them, but never bites. I usually put her down when she does this, and say no. When she meets other dogs she will bark like crazy and show her teeth if they get near her. Once she has been around them a bit, she does good and will play with them, sleep with them, and she is not food aggressive at all with dogs or humans. She seems like a very dominant dog, she always has her tail up and while she doesn't bare her teeth or bite, she does hold her ground and bark at them. I am at a loss of what to do with her to get her used to other dogs, other than introduce her to them.. it seems that once she is around them a while she is fine so I am not even sure it is a problem. I have had her about 5 months and introduced her to 4 dogs so far including a border collie, corgie, shih tzu, and german shepard with no problems besides initial barking and growling. We don't know much about her past other than she was extremely neglected, malnourished, matted, had scars, and was missing teeth from dental disease when the rescue group found her. What are some ways I can introduce her better to dogs and get her to be more friendly?