Hmm...I have a guy who's 2.5 lbs. I haven't found any issues in regards to him being 'safe' in his interactions with our other, larger Yorkies, but then again, I couldn't fathom throwing him in with our 50 lb hound either.
The most worrisome issue with him is not anything regarding his interaction with the other dogs, it's other things. He gets underfoot a lot and is so tiny. Anytime you're doing anything other than just sitting around, we have to put him up in his crate. Can't let him go upstairs with us because he can slip through the banisters. Have to really hover over him when taking him out because of the threat of hawks and owls. He can't sleep on the bed, because if you rolled over on him, or shifted and he rolled off the bed, it would kill him. And he seems to much more prone to hurting himself just doing things that the others don't have any issues with--jumping off the couch, stepping off the stairs wrong, etc.
I really do wish he'd gotten a bit bigger like his siblings.