18 inches is about right for a dropper, maybe a little shorter if you are fishing an emerger behind an adult.
I use a turle not to the bend of the dry fly hook.
5x is usually the tippet on my dropper but sometime you have to go to 6x. Also on a river like the Deschutes in OR you can get away with 4x. A lot depends on clarity of the water and how spooky the fish are. The line to my dropper is usually equal to my tippet if not a little smaller.