A Great Pyrenees puppy is highly trainable and while they are a LGB, they are great house pets also, which it sounds like you prefer. They are good with older dogs or other pets already in the home if they are brought in as a puppy. If you want to leave them out with your chickens you could after they are trained which would require time on your part, but any dog you bring in is going to require a lot of time with the chickens for exposure and to learn acceptable vs. not acceptable. Your Aussie will provide some training, but you cannot rely on that alone. Bear in mind, any breed can have an individual that is off. We had a Pyr once that could not be trusted around anyone, people or animals. Something was wrong with that dog - very high aggressive kill drive. Unless you go with a breeder, you never really know what you're getting from a shelter. There's obviously a huge cost variance between the two, whatever you get, remember your initial motivator is a friend for your current aging dog, chickens are secondary.