So? My family has rescued dogs that have been fought, beaten and starved. None of them have been vicious and i'd trust each and every one of them with my kids.
If you're not willing to take the stance to treat a dog how it's supposed to be treated, you shouldn't be allowed animals in your house.
Pitbulls are not outwardly violent creatures. They're favoured by outwardly violent people. The boxer is not a sponge that soaks up it's surroundings.
People like you are the reason so many dogs are destroyed for no reason.
Yeah I see where you're coming from with regards to being a responsible owner and guardian of an animal, but the OP said he had his reservations about introducing the two and was unsure of the dog's history - hence my use of the word "potentially".
If I was in the position where I was introducing two animals, I'd want to be sure I had a good understanding of both of their histories and their propensities for behaviour - be it good or bad.