A "Standard" holster is one that holds the pistol with the top to the front, or butt to the rear. The purpose of this rule is to re-emphasize the fact that butt forward holsters are not permitted in SASS.
After you've shot for a while, you'll begin to realize that transitions from one gun to the next are the essence of this game. You'll also note that much -- if not most -- of the time it takes to complete a stage in involved with deploying a gun and then, on the other end, getting rid of it as you move to the next part of the problem.
Often, the last gun that must be acquired will define the problem and limit your options for how the first gun must be employed. In other words, how you acquire and dispose of the first gun affects what it is possible to do with the second and subsequent guns in a stage.
There is a general principle among serious SASS shooters that one must be using both hands simultaneously to minimize the time it takes to make transitions. We don't put down one gun and then acquire the next -- we do both these things at once. This, of course, takes lots and lots of practice. But, that's what the essence of the game is. Minimizing the time it takes to get guns into and out of deployment.
For that reason, how you have your pistols holstered can help you solve transition problems or limit your ability to do so. I agree that off-hand-cross-draw rigs look really spiffy. They look mean and aggressive. But, be warned, they needlessly complicate solving transition problems and sometime completely prevent the shooter from using the optimum solution on a given stage. People have won championships with cross-draw rigs. But, most folks hit a wall a few years into competition where they cannot keep up with the good shooters who use the double-strong-side set-up. The cross-draw rigs present the further limitation of having to constantly be mindful of the cant of the off-side holster while drawing and returning the gun to leather. Double strong side rigs do not have this problem and afford the cowboy the benefit of the convention that says "the shooter must be allowed to holster their pistol." Please don't let that comment get us off track. Simply put, the double-strong side rig will ultimately allow you to be a faster, safer, and smoother shooter.
Cartridge loops are just clutter on a gun belt. You can't leave brass in them for any length of time without corrosion of your ammo. An the placement of loops on gunbelts (i.e. at the back) is not helpful for actually acquiring ammo during a stage. Reloads are common in SASS and you must have a rig that prepares you for that eventuality. However, for rifle/pistol ammo, a small slide is all that's required. You can put the rifle/pistol ammo slide anywhere and move it during a match to optimize your solution to a given problem. I like wearing my rifle/pistol slide, which holds five rounds, over the last two shotgun loops on my shotgun belt. You can't do that with a shotgun slide, but it's perfectly legal to do it with a pistol slide.
The only important thing on a gunbelt is that it be lined with rough-out on the back (inside) of the belt. This will keep the belt in position once you get it where you want it. All serious gunbelts and shotshell belts are lined in this way (Mernickle, Kirpatrick, El Paso, etc.)
I have had excellent service from both Kirpatrick Leathers and Mernickle Leathers. Each has a great selection of rigs.
You must have holsters that reveal the entire trigger guard and much of the top-strap of the gun to be able to adhere to the "two things at once" protocol of serious shooters. If you buy any other kind of holster (i.e. the "traditional" styles), you will end up having to get a new set before you wear out the first ones so that you can move more efficiently.
So, call Kirpatrick and order a double-strong-side Longhunter rig with Shotgun belt. The rig comes with ammo slides.
You won't be sorry. It will last you a long time. You will be able to improve without having to spend more money later.