My yorkie is 8lbs and gets along well with my parents' two staffies and labrador. He prefers the female staffie. When I got him, it was just the female staffie and labrador, but now they have a young male (not even a year old) and Cash gets a bit aggro when he is around.
The young boy, Magnum, thinks Cash is the most interesting thing on four legs and follows him everywhere. I don't let them play alone since they have had a few scuffles (I got myself bitten by breaking them up
). But Magnum is still young and a bit too dominant for Cash's taste.
It all depends on the temperament of the dogs. My parents have always had staffies and I grew up with one that bit my younger brother and viciously attacked other dogs (even our other dogs), but now they have the sweetest girl. Staffies can be really vicious, so just be careful and stop the play whenever it gets too rough. Give them a time-out when it gets too rough. They will quickly learn what the correct way of playing is.
Sorry for the long post.
Good luck.