Answer ASAP dog owners only please?

Have them meet on neutral ground (on a walk) and see how they react. But there is no way to "make sure they coexist peacefully" in the home. Ms. Terrier will insist on getting in the mastiff's face, and you won't know for a while whether mastiff will tolerate it or not (even if she does at first, she might get fed up over time, especially once she realizes this new place is now HER home too). I would suggest not leaving them together unsupervised, at least until you can tell if they've become good buddies.