Galvanized steel may be used for stair rails, coat hanger closet rods, animal pens, gates, antenna poles, etc. It can be used for water, but will eventually corrode.
Yes, the very popular 1/2" and 3/4" sizes usually cost less than even smaller diameters. This is because there are economic and production factors involved that make more difference than raw material and weight.
Before final assembly, the installer must remove the coupler and apply an appropriate joint compound.
Nipples are short lengths of pipe which have NPT threads on both ends (mipt x mipt) that fit any female iron-pipe-size threaded fitting of the same diameter.