Although the "more than about 100" scares me a little: won't this lead to 'em killing themselves all off eventually, if patrolling a particularly tough AI world?
Probably so, but then that's probably how it should be. My concern is still with whether or not gatlings will cause ships to go into threat status by themselves, and whether they will alert adjacent planets. Freeing other ships might not be so bad, since the gatlings will probably just kill them as well. Alerting adjacent planets could be a good or bad thing, depending on which direction your empire's expansion plans are headed. I'll just have to play with it and see. Looks like it'll be pretty great, although as far as I can see from theorycrafting, it's probably going to make the dyson sphere even more of a powerful ally.
You don't need to worry so much. Minor factions (except maybe for the devourer golem, but I think that was fixed) cannot put planets on alert. Also, they cannot free ships by merely firing upon them or getting close to them, AI ships will only be alerted in those cases. However, they
can free ships by killing a guard post, because that would lead to all the ships defending that guard post getting freed. As you mentioned, that wouldn't be so bad because the gattlings would probably kill those freed ships pretty quickly.
Also, someone reported that not only does the Dyson sphere stop spawning gattlings when it is on an AI planet and there are no humans around (which is intended behavior), but already spawned gattlings will stop firing on AI ships if they are on an AI planet and there are no humans around (not intended behavior). Can anyone confirm this?
EDIT: Found the original post for the issue in the second paragraph:
http://www.arcengames.com/mantisbt/view.php?id=2680#c11341