• Can I add the mod mid-game?: Yes you can. It will remove any weapons, ammo, turrets, etc. from the game (including blueprints, recipes, etc.), kill all existing biters, and prevent them from spawning. When using Pacifist with other mods, I recommend that you create a backup of your save file though.
• It breaks when I combine this with mod X: This is bound to happen, as Pacifist basically removes "everything that's a gun", "everything that is ammo", and so on. Mods that rely on their custom guns, ammo turrets, etc. might become unusable or prevent the game from starting. Please report any incompatibility in the discussions here or on GitHub