Not to mention, you have two ADULT dogs, and a shotgun, while you're proposing a single puppy to OP, and you don't even know if they possess a firearm. In addition, handling and remaining healthy and well are two different things, and I wouldn't want to put my dogs in danger just because they...
To answer your original question, I wouldn't get any more chicks until the area is predator proofed, but when you do that, maybe some silver laced Wyandotte, or a buff Orpington
LGDs are not always the best solution for every situation, not to mention Australian shepherds are not even LGDs to begin with, and going up against a fox can be dangerous, plus they need proper training in order to be around chickens