The original poster asked the basic questions -and sure got a variety of answers....
I'd refer him to McCracken's old thread near the top of the page "on fighting shotguns".... Yes, it's five pages but well worth a thorough read....
Of all the posts on this thread, post #5, and post #33 stand out for me. All of my experience with a defensive shotgun was in police work - where I pretty much had to learn on my own the stuff that you can gain reading McCracken and the two posts I've cited....
Like many who've actually carried and used a riot gun for some years I always kept mine as simple as possible - a four shot tube, a bead sight, 2 3/4" 00buck rounds... The only accessories I ever added were an elastic butt cuff with a few additional rounds and a sling, in case my hands were needed but I didn't want to drop my weapon on the ground.... On the street speed and maneuverability are life itself in a close quarters encounter. Hope I'm never needing that sort of skill ever again...