I'm working on a mod and want to set up 12 turrets that fire on any Actor that is not in a certain faction when inside their defense zone. Unfortunately, I don't know how to set this up. Any tips? Also, is there any way to make them fire only when they have a direct Line Of Sight on the offending target? I'd like to avoid using any FOSE functions if possible.

Did you check out the GECK site? In the tutorial to make your own vault it shows how to set up turrets:

http://geck.gamesas.com/index.php/gamesas_Tutorial_Non-NPC_population#Manipulating_Turrets_With_Terminals

Then I think it is just a matter of using which one you want (or creating your own type if need be) with the correct faction. You can also set up terminals to allow them to be deactivated/target anyone like they do in the regular game if you want. Otherwise, you could just have them hostile to everyone, though you would want to be sure the turrets are in the same faction if they are close enough to one another so they don't shoot each other. =)