1: Yes. Shot size is the diameter of the pellet (or rather the overall average since individual pellets can vary a bit.) And shot is shot, no matter the guage.
2:Steel shot has progressed pretty far since the ban on lead for waterfowl hunting. There are some great loads out there, but you do have to realize the lmitation it imposes. Steel is so much cheaper, and if you learn to use it right then there is no problem at all. Tungsten/iron matrix, heavi shot and all the "heavier than lead" DO preform and great at that. The biggest down side is cost. You pay a few dollars more for 15 less rounds.......it can get pricey depending on what your shooting. Turkey loads aint that bad since you dont burn that many in a season (or at least you shouldnt or you need more time at the range
.) Goose/duck, however? You could burn a small fortune in one sitting.
3:The copper plating helps reduce pellet deformation which leads to better patterns. Also the plating allows for slightly increased penetration on game. Generally used in Turkey loads and high preformace pheasant. Worth the extra dough in my opinion.
Hope this helps.
Brian M.