I'm sure I read on another post that Vahn tweeted that it would affect SMG'a and Assault rifles as it's a mix of both, hopefully someone can post the link for you as I'm not 100% sure
You *do not* need to level up the Peacekeeper, to get Diamond on the rest of the SMGs. All will go Diamond when you get them Gold.
Anthony: The Peacekeeper does not affect diamond progression for the 6 core submachine guns - they will function exactly as before. If you earn Diamond Camo on the core 6 SMG's, when you earn gold for the Peacekeeper you will then get Diamond for it as well.
It works like this:
The gun is in the SMG class. In order to get the standard 6 guns diamond, you only need to get those 6 gold. The Peacekeeper is in that class, but not part of the diamond progression.
To get diamond on the peacekeeper, you need to get it gold, as well as get the rest of the guns gold.
So if you already have diamond SMGs, when you get the DLC, your 6 SMGs will still be Diamond, but the Peacekeeper will not be. Obtaining the gold camo for the Peacekeeper will unlock the diamond camo for it.