As a guide boxers naturally tend to quieten down at about 3 years of age.
We have 2 boxers, 1 male & 1 female that are both 3 years old, that recently have started to quieten down a little (I do stress a little...LOL).
You have to remember that boxers are very active (yes, some would say hyper) dogs, hey but that is just another reason why we love them.
Neutered early or neutered late it makes no difference to how much energy they have.
As I said previously about neutering,
The only bahaviors that change are the ones that are influenced by the male hormones. Things such as interaction with humans & playfulness do not change.
As for what mix of boxers to have, well you'll find people in here that have every combination. I think that you can have any mix especially when they are neutered / spayed.
As for having 2 making them quieter, in our experience I guess it did, but I think purely because they have company when left alone. They still have their moments though....LOL LOL