Are you concerned about the big dog attacking the smaller one? Or your first dog feeling ...left out?
I have very limited knowledge of such things, but here is what I would do (have done) to minimise any potential problems.
Take your new Mastiff for a big walk first to work off some energy, then allow your dogs to meet on neutral territory - not in your Foxy's yard. We did this out the front on the footpath.
Let them have a good sniff of each other's behind's - don't let them sniff each other's faces at first. When all that goes well, take them into your yard.
Still treat your Foxy like it is higher up the chain than the new dog ie: she gets her food first, she goes though the gate/door first (AFTER you of course) etc, so that the new Mastiff knows that the Foxy is in second in charge, after you and your partner and children.
I know there are a couple of people on APS who are knowledgeable about dogs and training, hopefully one of them will see your post, and be able to help you more.
Supervise a bit too, a Mastiff, even playing, could inadvertently hurt an elderly smaller dog.....
Then again, you could find, like we did, that your older dog reverts to being a playful puppy again when it has a friend
Good luck