What is the perfect breed for a companion to a rottweiler?

We have a 2 year old rottie female and are looking for a second dog of a smaller breed as a good play companion for our cherished Karma. She is great with other animals, but we need something small and sturdy, because we really don't get into bony small breeds, we have a small home and 2 kids ages 9 and 14. We don't care for excessive barking and aren't a high energy family. What would be the ideal breed, (other than a mutt from the spca which we have considered)for us? We think maybe a pug, perhaps? Oh, and the less drool, the better!