I don't read the dog board too often so I just saw this today. Have there been any more instances since your original post? The article someone referred you to was great. One problem I see here is the size difference. Your boxer pup has no way of knowing that the amount of force he could use against another boxer could be lethal to a chihuahua. I would advise that the dogs always be kept separate when no one is around to supervise. I know that's a pain, but best to avoid an injury to your little dogs.