Check out the reloading forums on shotgunworld. Especially with the mecs, there are a lot of very knowledgable folks over there.
The AA's are Ok for reloading but they can have issues with hull buckling, especially if you get a little too much stuff in there. Remington Premier ( real brass bases) or even Remington Gunclubs (steel bases) are much better 20 gauge hulls.
Unique is a good 20 ga. powder. At least for target loads, I couldn't say about field loads. I use Alliant 20/28 or Hodgdon Universal Clays - neither of which have been redily available for some time. Supply is supposed to be easing, but we'll see.
For 20 gauge wads I use RXP 20's if you can get them. I have a case of 20 gauge Windjammers hidden in the closet when I finish up the case of RXP's.
You are correct in that there is no 'load work up' in shotgunning. Follow the recipies. Look at the powder mfg. website for loading data and make sure the hull, primer, and wad combo are listed for your powder. The powder co's tech people can be helpful if you want to try a substitution. Tell them exactly what it is you are trying to do and they will generally tell you if it's ok or not.
Also - bushing charts are just a guidline to help you with ordering the more or less right bushing. Always verify your drops with a scale. The way you operate the machine, how well it's mounted, single stage or progressive, etc. all have bearing on the powder throw weights. Generally the single stage machines will drop a bit heavier than progressives because you cycle the handle many more times per given shell, so the charts tend to make sure they are 'safe' with single stage loaders. Also, very generally specing the bushing tend to drop a bit light, so if you are ordering bushings, order one or two to either side of what you're looking for.
SAAMI pressure for the 20 gauge is 12,000 psi. Most reloaders tend to like recipies that leave some margin on this, maybe 10,000 - 11,000 psi. Too low a pressure and you may not get good burns or you may get 'poopers' in cold weather, too high a pressure can get you a kaboom.