How can I make my new dog get along with my other dog?

I just recently adopted a 9 month old staffordshire terrier-pitbull. I already own a 4 year old chihuahua. She’s been with my brother since she was born but I came to own her a year ago. She hasn’t been well socialized and when she was with my brother he also owned her brother, so there were 2 chihuahuas always together. She did not like her brother too well. He would always bully her for her food. Since I have owned her she has adapted to my lifestyle well and seems happy, although she is an EXTREMELY picky eater and can go days without eating.

Anyway, she doesn’t seem to like the new puppy. He is very playful and tries to play with her. He does try to flip her over with his paws, which I don’t like and she doesn’t like either. There are also times when he sniffs her and she growls at him. Also, sometimes she’ll let him sniff her and sometimes she won’t. Every time he approaches her she is fearful.l She also does everything to avoid her and won’t go near him willingly. Only if I am by him and I call her she’ll come to me, but won’t through the side that he’s on. How can I make them get along? I walk them together and they do fine, but how can I make them get along at home?

P.S. He is neutered and she is not. I’m sure that this has something to do with her attitude, but how else can I make them get along?

Thanks!