Hello, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
This is obviously a tough situation, isn't it?
In my experience, kennel cough rarely, if ever, turns into pneumonia for those dogs who contract the disease (even older dogs) but I can understand your not wanting to risk the chance with your older girl.
While kennel cough can be contagious, it doesn't necessarily have to be; it's somewhat similar to a cold in a human: just because you're exposed doesn't necessarily mean you'll develop symptoms. But, again, I can understand why you wouldn't want to expose your other dog.
I wouldn't put your dog on prophylactic antibiotics on the "off" chance that she'd develop kennel cough; this wouldn't be considered an appropriate course of action especially since the underlying cause might be virus. As you know, viruses don't respond to antibiotics so this won't be a guarantee that she won't develop symptoms.
The kennel cough vaccine may help if this is a bacterial infection but, again, it won't be effective if a virus is responsible for the symptoms seen. And, for the vaccine to be effective, it should be given at least one week prior to possible exposure.
I don't think it's too risky to adopt this rescue dog but it might be advisable to wait and bring her home (if at all possible) until she's symptom free. Again, this won't necessarily guarantee that your dog won't develop signs at some point since the rescue dog could be a carrier and still spread infection. But, this could be true for any dog which your girl might meet...either on the streets, or dogs who belong to friends or family or neighbors. It will be extremely difficult to completely protect your girl from exposure to such an infectious condition since we can even bring home pathogens on our shoes or clothes.
If you do decide to bring her home, then I'd try to isolate her and prevent contact with your dog as much as possible. Wash your hands after contact with the rescue dog; possibly consider a chance of clothes as well if you keep her/him in a separate part of the house. This may not be possible, of course, which is why it will be best to wait to bring her home until she's not showing any signs.
I hope this helps but, again, my apologies for the delayed reply. And, I hope it works out with the rescue. Deb