No small dog will be safe out there with coyotes or larger dogs. a guardian dog needs to be large enough to intimidate coyotes, at least, and be likely to survive an encounter.
I agree that large terriers would be very difficult! The Chessies and the German Shorthair that I raised from puppyhood learned early to ignore the chickens, but I had them at the house, not living out there with the flock.
Having an actual LGD living with the flock, or better yet, two of them, will be an expensive and long term project, and include good fencing.
Electric fencing is much easier to manage, and much less expensive!
Mary