You can't use non-optimized custom weapon mods with FWE? Not even the ones that just add 1 or 2 guns?
You can use weaponpacks with FWE sure(just no weapon overhauls like CFWR). But it's better if there are specially designed FWE versions or patches, otherwise the stats like damage and spread could feel a bit off compared to other FWE weaponry.
And like I said in the mainthread, we're adding several new weapons to FWE in 5.1
But FWE has with all DLCs and the WMK variants almost a thousand weapons entries just fyi