First, I am confused. Why won't the puppy come home until it is 8 months old? Will it be fully trained before you get it?
Second, everything depends on individual animals and their temperaments. My GSD happens to share a room with my chinchillas. Chilli is a tease- the boys tag team her and nibble on her ears if they poke into the cage. She looks at them, she hollers at them, but my chins are used to a certain amount of activity around them. They always made a big deal when my older cat came in the room because they knew he was scared of them. They would alarm call, and when I went in, they would be sitting up at the front of their cage, just looking at me. Dickens would be somewhere in the room. My younger cat went through a phase where we would have to close the door at feeding time or she would jump into the cage before we could catch her. She just hung out, looking around.
So much just depends on the dog. I have never heard of the bacteria thing, but reality is, that we carry stuff in on our feet every day that could make any of our pets sick. I would not allow the dog free roam of where the chins are until you have done some on lead work and carefully supervised interaction. I don't see why having a chin prevents you from getting a dog. You just have to assume a higher level of caution about cage locking, etc. A ferret on the other hand is a different story.