You can have a longer moderator made, then jus have it made so that your barrel goes extends from the frame into the moderator.
This is a 10" ldc on my talonp, and when Neil Clague made it, he asked if I had any barrel to cover, but I do not do to the mircolite bullpup kit, which sits the barrel back 5".
On a normal talonp if you add a 18" barrel you're going to have about 5" of barrel sticking out of the frame, so you could get a moderator made to cover it up, basically a shroud, if you'd like to.
I only have a 16" but I wanted maximum quiet and the 10" provides that.