While you'll probably be fine, I wouldn't chance it especially with the upper. If the face of the receiver isn't squared, the problems that can come from that will be amplified. A lot of great lighter weight billet uppers out there, that I'd suggest you go with.
The gas ports in the Lilja 18" barrels are sized specifically for an 18" barrel. I'm the one that ran the original group buy that got Lilja in the Grendel game. I'm the one who gave them the gas port sizes after getting the correct specs from the appropriate sources. (That group buy was an incredible learning experience for me.)
That being said, I always suggest adjustable gas blocks, particularly the SLR blocks.
On top of that, 18" do benefit from longer gas systems. However, an 18" with a rifle length gas system can be very finicky. Precision Firearms sells an 18" barrel with intermediate gas system, which will come with the appropriate gas tube.
I think you'll be very happy with either a PF or Lilja barrel.
IMHO an 18" Fluted barrel sounds like it would be perfect for you. I'd suggest a light weight 2.5-10x scope.
Here's my latest build.
PF Bartlein Barrel 16" Fluted, Spikes billet Upper forged Lower, NF 2.5-10x24 mil/mil in an ADM Recon mount.
I'm currently working on another group buy with Lilja, as I've got a serious itch for a Grendel SBR.
Grendel Short Barrel Group Buy Orders