This could be a tough one, because you want to protect the little guy at all costs, but you don't want to do anything that is going to cause any animosity between the two dogs. Especially since the Staff could really hurt the Frenchie when he grows up.
First of all, I would make sure that any play between the two puppies was carefully supervised until the Staff grows up a bit and gets some training under his belt. If they are not playing, make sure the two dogs are separated.
Now, particularly if the Staff enjoys playing with the Frenchie, as soon as the play gets a bit rough, immediately pick the Staff up and remove him from the room. Close the door and let him calm down. After a few minutes, return him to the "playground" and let them interact again. Once again, the second the Staff gets too rough, scoop him up and remove him from the room. Eventually it should dawn on him that if he acts inappropriately toward the Frenchie, he will be taken away from him.
That's alot for the advice