my boys have played with australian shepherds, big labs, other yorkies, rat terriers, mutts, etc...and we've never had an issue. with the bigger dogs, I make sure that they're always watched, and that if either the big or small dogs need a break, they are separated. As long as you're careful, I would say there's no issue with letting them play
I would be careful about the big dogs playing rough with the small ones, but all you need to do is slightly intervene when play gets too rough...they will eventually learn to be gentle.