The vet never gave a diagnosis, but it was obviously respiratory, whether it was pnuemonia or not I don't know.
i did get a replacement at the pet store since I'd purchased Mac just over a week ago. I named this guy Lockheed, and he is hands down the most affectionate pet I ever had. I don't know how, but he learned his name in just over a day (or maybe he just comes no matter what noise I make, but it doesn't seem that way), and he seems to be fully litter trained. He's older than Mac was, but I don't think Pet Smart is in the habit of training their animals, are they? Anyway, the guy follows me around everywhere when he's out of the cage, and even when he's off exploring, he comes running back when i call him over for play. Hopefully that won't change once his quarantine is over, he gets fixed, and moves in with the girls. (Oh, and I put together another cage for him, the one I had for Mac was way too small)
I have another question about one of the girls, but I'll start a new topic to keep things easy.