Mid length is 16" bbl. with rifle [20" ] gas system. If I take a 24" barreled upper and cut it to 18", I have a smoothe running 18" rifle.  Right?

 This armalite top I plan on having cut has a flattop and front gasblock with a minni rail. If I use a '40' rear flip up sight what front sight should be used on the picitinny front gas block?

No, midlength has a different bolt face to gas port distance than either the "CAR" or 20" rifle lengths.
The difference is that midlength gives the same proportion of barrel forward of the sight tower as on a 20"

If you cut the 24" to 18" it will look like the 11.5" upper, a stubby barrel forward of the sight block. You may run into trouble since there's less barrel forward of the sight tower but the gas port is sized for the 24" bbl. You can drill out the port though.

Why not just take the barrel off and buy a shorter one? It would probably be a very similar cost to having it cut, crowned and drilled...