While my English Shepherd loves small dogs (the smaller the better, in her opinion), especially males, if I were to ever get a Chihuahua or something similar in size I would not leave them unsupervised. The big size difference really ups your chances at injury, IMHO - even if Quinn simply forgot where her feet were or corrected a Chi, there could be injuries. Supervised - sure, she'd love it, but I wouldn't leave them unsupervised together.