We got Sam the mutt pup when he was quite young and only 12#. (hard to believe he was ever that small!) Maggie is a high prey drive kinda girl so I knew from the beginning we were going to have to be very careful. Sam was crated when we weren't home. He was in an x-pen when we were home and not actively playing with him and when we were playing with him, Maggie was muzzled. And i just didn't let them outside together till Sam was 20# or so and even then, Maggie was muzzled and extremely supervised. And we kept these rules till Sam was 30#. That seems to be the magic number for Maggie.
I'll bet you can get through this with muzzle, crate, separate turnout and lots of supervision. Good luck.