I'm not really a dog person, and the only time I've ever had an interaction with a pit bull, it was awesome. When I was with my last ex, I met her huuuuge pit bull, and she was the sweetest and most happy dog I've seen. Unfortunately her passive pit allowed her family's bikes to get stolen... Apparently the thiefs befriended the big dog and managed to just hop the fence and take the bikes..
I'm sorry to hear about your shih-tzu.. I don't trust any stray dog. We brought in a stray retriever who seemed super friendly to us, but he got a hold of my favorite cat and killed her... I was about 9 years old when I saw my only cat being violently shaken to death by a dog we tried to help.
I still think dogs are amazing animals, and I think the way the dog turns out is dependent on the owner. If it's a stray, then you should notify animal control immediately in case it's dangerous to you or your pets.