THe difference between a slat and a leading edge flap is that a slat has a slot that opens when the leading edge is deployed while a leading edge flap is just that, a surface hinged to move down and create camber.

I believe the MD-80/90 series have leading edge flaps. I could be wrong. Just rode a -8x last week and I don't remember seeing the leading edge slot, just the drooping flap.

Leading edge flaps can be done relatively simply by hinging the surface on the bottom skin and using overlapping top skin to cover the gap as the flap is deployed.

As far as the airfoil, you didn't mention the airfoil number/name in your other thread. WHat is the full scale airfoil? Not so much to use the same airfoil, but to find an airfoil of similar thickness to work for you. Using an airfoil of similar thickness will make it easier to mate the wing to the fuselage in the scale mating area.