What is a good, little, dog to have?

Alright, I have and italian mastiff and I want a little dog too. But my mastiff is so good. She doesnt barely bark, or chew, she is potty trained, and just great. I want a really little dog that is like my big dog only tiny lol. I mean it can be a mix it doesnt have to be full bred I just dont know what kind of dogs will be a good fit for my mastiff that will get along with her. Any advice will be great. Thank you!