Could This Hurt My Pug In the Future?

I've posted a lot about my Pug puppy (6 months) and my girlfriends chihuahua not getting along. My pug loves to play and he's an energetic little pup, which I know is typical breed behavior. However, the chi is terrified of him. When he's not trying to play with her (just typical puppy play) she's fine, but when he goes towards her she growls and is now starting to become overly aggressive. I have been discouraging him from playing with the Chi, however, this is his only real interaction with other dogs right now. Its too hot in NYC for him to be out long enough and go to dog runs and whatnot, so he really only gets to interact with the chi. Is it going to hurt his behavior towards other dogs in the future if I continue to tell him 'no' when he tries to play with the chi. He's definitely frustrated because he loves to play but only gets to play with me. I know puppies need to be socialized, so what do I do? The chi is not going to change. She only gets worse. She growls angrily even when he brushes up against her. (She was not socialized as a puppy and hates other dogs). I'd love to take him out to a dog park and let him get his playing out - but how can I do that when its 90 degrees and humid out? Any suggestions? I really want him to grow into a dog that gets along well with other dogs.