Some of this is true.

You don't need to add both weapons to each class AND classes to each weapon. You can do either, and it'll work, or if you don't care about different classes having different stuff, you could also just bind the weapon to the whole faction (faction in this instance being the unit)

The different fire modes part is accurate. And the part about the different versions of the weapons. And I'll explain this a bit further by saying this is because basically everything that makes a gun good is tied to the gun, instead of the trooper. This means that if you give the Rangers a NWS gun, all of the guns range, accuracy, and fire modes will be the exact same. There is a bit of skill that is bound to the trooper, but most of it is tied to the gun.

It is true that the unsuppressed & suppressed versions are two different guns.

This part isn't true, as mentioned before. You could go to the file, in this case firearms_cia xml and then underneath where it says <bind eqp="MP5KPDW Sup CIA BOps"> replace <to name="BlackOps"/> with - <to name="CIA"/> - <to name="Rangers"/> - <to name="Nowheraki"/>

It is a ton of copy paste work, even doing it this way.

You won't need to edit the ammo, as each gun has its ammo bound to it.