In addition to our 2 boxers we have 2 small breed dogs, a maltese and a bichon frise. I love these little guys to pieces, but would doubtfully have another of either. They require a lot of grooming for their long hair. But as far as temperment and getting along with the boxers they are great. But each dog, no matter the breed, is different of course. If you are not crazy about all the grooming and taking care of long hair, maybe a boston terrier, pug, french bulldog, beagle, or bassett hound would be better?
I have never had a problem with agression between any of them, but have always heard it is on the safe side to stick with male/female pairs. Again I think it all depends on the dogs' personalities.
It is great that you are trying to research before getting your new pup. I hope you find the perfect fit for you family!