If the Laegar's arms really are made for 17.25" you will be fine running 18" shocks. Typically 3/4" either way, + or - is fine. It's not PERFECT, but it will work without any issues, in my opinion. Of course, without having the bike in front of me, I cannot give you a concrete "YES" or "NO", but as long as those arms ARE indeed built for 17.25" you'll be fine. Dune valving typically is a little lighter valving than any other type of setup, but being completely honest, the difference is so minor, that 99% of people wouldn't notice.
Buy the shocks, run them, and go beat the hell out of it.