Hmm maybe a Jack Russell or some other small terrier type with crazy critter prey drive?
I'd be nervous about keeping a cat outside, they like to roam and wander pretty far sometimes... While lots of people in the country do it, letting them be outdoor can significantly reduce their lifespan. BUT that's just me, I'm super paranoid lol as coyotes get cats out here so no way mine stay out.
Have your shepherds been around cats? That's something I'd worry about too.
~Ollie vom Grunenfeld HIC(8/29/2012)WGSL/WGWL/DDR cross - herding dog in training!