You don't know how any dog will react under stress. Even a dog from the best breeder will overreact at times. The best thing to do is to keep the two separated until you know the dog well. Once you know a dog you can really read them and know what they'll do and how they'll react.

If you get a dog from a shelter, you can feel like you helped an animal considerably instead of just buying a dog that you know would be fine just the same.