Some unique weapons without a common counterpart (for example That Gun and This Machine) receive one.
Some weapon mods can be applied to multiple different weapons. But many more are weapon mods newly added by WMX. Some existing weapon mods (from the base game) are newly applied to weapons besides their defaults. Unique weapons can be modified with the same weapon mods as their common counterparts. ALL weapons get the maximum of three applicable weapon mods this includes unique weapons.
Weapon Mods Expanded (WMX) for Fallout New Vegas expands upon the base game's weapon modding system in the following ways: I like Obsidian's take on it enough that I wanted to add to it. My Weapon Mod Kits (WMK) mod for Fallout 3 was the inspiration for Obsidian's implementation of the weapon modding system in Fallout New Vegas something which I am quite proud of and flattered by.