Pitt bull, sheltie and papillon living together?

I have a 3 1/2 year old Sheltie female and a 2 year old pitt bull male, they have been getting along very well since he was a pup. No aggression problems at all towards each other. The male plays gently with her and babies and my sisters less than year old rat terrier. I am looking to get a papillon pup.I have owned dogs for years, but never a small dog like a pap. I watch my dogs like a hawk and they dont play together when no one is around. My question is to someone with papillon experience and who is no stranger to the ways of Pitts or other large dogs...do you think we could live togetther without harm coming to the papillon? Any thoughts and experiences are welcome!