The only ammo that I had an issue with in my G21SF was some jacketed semi-wadcutter bullets, but they also had a very short OAL. Other than those, no others to report. I have ran 185gr, 200gr, and 230gr weights from several different manufacturers with no problems.
Accessories are a personal thing. I had to go through some trial and error to figure out what accessories were worth their weight.
-Probably the most important item is ammunition. If you don't already reload, you may want to look into it. You can save yourself a lot of money by loading your own. The cheapest ammo at my local Wal-mart is roughly $20 for 50 rounds. My cost is $8 per 50 rounds. I started out using a single stage press. My start up cost was right at $800. The press had paid itself off within the first 6 months of use.
-I would recommend a good set of night sights. I prefer Warren Tactical sights or Heinie straight eights.
-If you plan on carrying it, a good holster is a must. I carried my G21SF in a Blade-tech NANO IWB. I also used a Galco summer comfort IWB, but did not like it as much as the Blade-tech.
-Its good to have plenty of magazines. I like to have at least 6 mags for my carry gun. I can run three as range mags and use the other three for carry. I like to have plenty of mags on hand, so the more the merrier
-I like the extended slide lock lever, but not for use as a slide release. I find that it is easier to engage when you are manually locking the slide back for admin purposes.
-The back strap channel plug is optional, but I use them. I have heard both yays and nays on them.
-I have a stainless steel after market barrel for my G21 so I could shoot my lead reloads. My barrel is from Lone Wolf Distributors and has been reliable so far. They normally run around $100.
-The stainless steel guide rods are optional as well. Some of my Glocks have them, some don't.
-I am using an extended magazine release on my G17. I cut it down to where it was mid way between factory length and the extended length, so far so good.
-I am testing out a Talon grip on my G17 and I like it so far. The sand decal grips are also good if you find that the frame isn't as grippy as you would like.