On the shotgun, every instructor I've read or taken a class from isn't a big fan of the pistol grip -- for exactly the reasons you probably encountered. It's hard to aim and, if you do aim, it's hard to manage the recoil. If you're actually using the bead sight, you're holding the shotgun in a position where you might as well have a shoulder stock.
On the Mossbergs, the pistol grip makes it hard to reach the top-mounted safety. It's a bit of a shame -- the placement of that safety is genius with a regular stock.
I have been really impressed with the MagPul shotgun stock. It's ugly but very ergonomic and adjustable. If you're trying to save length, just take out all the spacers.
One piece of advice I read but never tried was, if you are using a pistol grip, hold the shotgun at waist level. Yes, it looks "Action Movie" but it puts the right arm in a better position to control recoil. Let your elbow be the fulcrum. Since this makes it impossible to use the bead, a laser might also be a good investment.