Depends entirely on the temperament of the Staff bitch. Often two bitches do not get on, although this may not occur until the younger one reaches the age where she wants to be 'top dog' .
There is only one way to find out, provided of course the Staff bitch has shown no aggression to other dogs whenever she has met them. If she has been aggressive or dominant over any other dogs, then I think you would be in for problems, particularly as Boxer bitches can be dominant.
Ask if you can let the Staff meet the puppy before you commit yourself, and provided they meet under strictly controlled conditions (not on your bitches home territory)
then you should get some idea of her reaction. If she is prepared to play and allow the Boxer pup to play roughly with her (as Boxers tend to do!) then you may be ok.
I would suggest perhaps a dog pup might be a better bet, and then you can either have the bitch spayed or the dog castrated when old enough to avoid any unwanted puppies.
This is of course provided your Staff is not aggressive with any other dogs.
If things go ok and you decide to get the Boxer puppy, please ensure very careful mixing when you get them home, and don't leave them alone together. Get a crate for the puppy to sleep in so it doesn't pester the older bitch and so they can both get some peace.