we got Louis at 8 weeks. We couldnt set him down outside because of the immune system and he needed certain shots and so on. We used purell if anyone just wanted to pet him. It was hard to socialize him with other dogs because of this ( we don't have any other pets) but it could be ok with you since you have another dog and other known dogs that you aren't afraid of them to be sick or anything.
Louis is 11lb but was very little when we got him 3 lb. He has made it so far.
Best of luck! and Congrats on your new baby boy... I'm jealous
