I've been trying to get it set like this:
Autoturrets can kill zombies
Autoturrets cannot kill npcs

With TurretsIgnoreScientists disabled:
Autoturrets kill zombies
Autoturrets shoot npcs infinitely without doing damage.

If I enable TurretsIgnoreSicentists, then nothing happens at all.

Is there a way to get turrets to shoot zombies but ignore scientists instead of draining all the ammo on them??

My config: https://pastebin.com/Xp5mtD9V

your plugin can bypass the TurretsIgnoreScientist flag using the CanEntityBeTargeted hook

I didn't explain correctly.  To put it simply.

2 plugins installed. BotReSpawn & ZombieHorde.

Is it possible for AutoTurrets to shoot only zombiehorde but no other npcs?

Currently in the config I can only get 2 results right now:
Autoturrets ignore both zombies & scientists (TurretsIgnoreSicentists) or
Autoturrets shoot and kill zombies, but also shoots scientists that take no damage and drain the turrets.

Ideally, the latter option should kill zombies, but ignore scientists.

Is it possible to achieve this?

they would need to use the hook above 

TurretsIgnoreScientist will block the targeting otherwise

private object CanEntityBeTargeted(ZombieNPC zombieNPC, AutoTurret entity) => CanBeTargeted(zombieNPC, entity) == null;

you can put that in the ZombieHorde.cs above line 173 for version 0.6.8