We've got a 2 1/2 lb Yorkie and a 14lb Cairn terrier. The Cairn was rough at first but he's learned how to "play gentle". He clearly doesn't want to hurt her because if she yelps (and I've seen her do this when he's no where near her!!!!!!) he runs over to her to "check on his lil sister"!!!
I've also seen him bring her a toy to play tug-o-war with. Braxton is strong enough to break Calliope in 2 if he wanted to but he holds the toy and lays down so she can tug on it. Sweetest thing you've ever seen.
Just teach them how to play together and don't let them out of your sight until you are comfortable with them. Once you practice with them, you'll know what the boundary is.
