Well, I have to disagree a bit...
Contrary to popular belief, shotshell reloading is not that component-specific or complicated.
I read an article once where a balistision,(SP?), went into why we should follow specific recipes when loading shotgun shells. He took a specific load from a respected manual, put it through a pressure gun. It performed whithin a couple hundred PSI of what was specified. It was a common type of hull, with everyday type of components. He then substituted ONLY THE PRIMER. The PSI went nuts, some dropping to where the velocity would have only been 900 FPS, others went to over 13,000 PSI! Now realize that the top pressure limit for 12 Ga. 2 3/4 inch shells is 11,500 PSI. Would that 13,000 PSI load have burst a modern shotgun? No, but it sure wouldn't do it much good. And the empty hulls would be cooked!
Again he went with the specified load, but this time only substituted the WAD. Again the pressures went nuts.
As for your question about hull identification, the absoulute best sorce for identifying hulls is in the Lyman shotshell handbook, the bible for shotgunners.
What SteveC said is good info. I routinely would load my trap/skeet/sporting clays loads with 7.5,8's, or 9's. But when I went bunnyhunting or for pheasants, I simply changed to 6's or 5's with the same powder/wad/hulls. You DON'T NEED high velocity to cleanly kill small game, just bigger shot.