Does that mod allow me to put 2 silencers, 2 extended mags and 2 night vision scopes onto Ratslayer?

Not quite. The guy who was working on it was going through each unique gun individually and manually setting them to accept mods for effects they did not get naturally - so no further mods on Ratslayer, but it allows you to stack up everything on, say, Maria. Not sure if all the uniques are done yet, but he had 5 or so finished last I checked.