Whew, you like 'em big, don't you? Are these show animals? If not, neutering will be some help, for either sex, but you will have to make sure you show as much attention to the "old" dog as you do the new. They are just as subject to "sibling rivalry" as humans.
You don't mention the Boxer's personality, but I would suggest that you introduce them in a neutral area like a park or some such so that the Boxer's territorial defensive feelings don't kick in. When I have introduced new additions to the "pack", I frequently use a wire crate to house the newer one in when the older one is out and then crate the older one at times to let the new one out to roam and play. It allows them to get the smell of each other and you to observe behavior so you can respond in a positive manner when the older dog acts nice around the newer dog. You will not want to respond too much to aggressive behavior from the older dog unless it endangers the younger. You want to reinforce the good behavior not the bad and negative reinforcement can be stronger than positive if you're not careful. They will have to determine their places in the "pack" for themselves.
My personal preference is female dogs for the most part as they, generally, are more even tempered, but not always. My two eldest females would sometimes get into it for pack dominance and we'd have to dump water on them to get them to stop. Just a caution.