NoDefaultMobArmour is a tiny coremod that disables the vanilla behaviour where mobs spawn with leather, iron, gold, or diamond armour pieces: they will have no armour regardless of difficulty. It will not affect modded mobs except insofar as they inherit from this default behaviour, nor will it stop mobs from spawning with tools (e.g. zombies spawning with swords, shovels, etc., or skeletons with bows).

It does this by changing the body of the EntityLiving.getArmorByChance method to be effectively "return null;", where it normally has a chance of returning either null or a reference to a piece of armour.

I intended this for use in modpacks where mobs get alternate armour, and I've licensed it under the MIT license, so, yes, you can use it in your modpack.