Long before the current trend in drop-leg holsters, we used some in Army Aviation to clear the armor on the seats in some specific aircraft. The one I flew had more armor coverage, and frankly even a drop-leg wasn’t going to work, so the shoulder holster was the way to go for me. Tanker wear shoulder rigs, as well as desk jockeys for the very same reason. Your pistol needs to be out of the way to do your primary job. That’s the Army though. Just because Big Army does it, or uses it, it doesn’t mean it’s really a good idea for you as an individual. There’s a lot of junk the Army uses to great effect that is just useless for the individual or small-group survivalist. Don’t ever base what you need on what you see the Army, or even contractors, using. The missions are entirely different. Buy and use what you need.
That being said, drop-leg rigs are great for wearing directly on your belt, or a dedicated gun belt, along with a knife. If you remember Trasel’s post a while back about gear he mentioned keeping your knife, etc. on your trouser belt, so you always have it with you, if you ditch, or just don’t have your web gear. Sage advice there. A drop-leg, or even a shoulder rig, does this for you. By using the right holster (that’s key there), you can have it attached to your person, and clear your web gear. If you have to ditch your web-gear, your gun and knife are still with you.
While most schools frown on shoulder holsters because of safety concerns, and the complexity of sidearm retention, in many cases it’s a good choice. Pilots have used them forever, and it’s unlikely you’ll face a retention situation in your own cockpit. Same with tankers. I remember a picture of a P-38 pilot in the Pacific that had the usual USGI  WWII  shoulder rig, with the shoulder strap also going through a mag pouch and survival knife. Not a bad set-up for his use, and worth thinking about for a lot of reasons. Not the perfect rig for a night on the town, but it obviously worked for him. Even what’s perfect in a schoolhouse training environment might not be perfect for you. The key is to go with what works for you.
Whatever holster you choose, if you have more than one try to keep to one system. If you’re using a Safariland 6004 , look at a holster with the self-locking system (SLS) for concealment, or go without [secondary] retention. What you don’t want is different retention systems to deal with. Using a 6004 with SLS on your leg, then using a thumbsnap for concealment, and using a level three retention holster for belt use isn’t a wise move. [For the sake if kinesthetic memory] you want to make the same movements each time to get the gun out.
Sometimes you can modify stuff to work. The Safariland 6004 is often the subject of some surgery which allows it to ride higher and much more comfortable for many. Sometimes you can get holsters that do many things. The USGI M12 [aka Bianchi UM-84 series] holster can be adapted to many different types of carry. I frankly don’t think too highly of that holster, but many think USGI means it’s the way to go. They’re cheap enough I suppose. I currently use an Eagle brand drop leg that the drop leg flap can fold over so you can use it both as a conventional belt service holster as well as a drop leg. It rides high enough to be out of the way in drop-leg mode, and low enough to clear gear. The full flap, with Fastex fastener means it’s secure no matter what I do, and the full-flap velcros out of the way to allow an open top configuration with a thumbsnap retention as well. It pretty much does it all for me from admin to tactical and it’s all the same holster, so training is simplified and it’s cheaper to buy one good holster than several different ones (though I always seem to buy several anyway). It’s doesn’t do concealed carry well, but most CCW  holsters are either non-retention, or thumbsnap, so again there’s nothing to re-learn in a fight.
As for slings, I said before the Israeli type has a lot going for it, and that’s what I use. You can beat it in specific tasks with other slings, but for all around lugging a rifle around and still be effective with the sling, they’re great.
While I agree on having different sets of web-gear for each rifle, I don’t agree on caliber/weapon specific. I think they should be universal for what you’re equipped with. That way all that needs to change is the magazines, and not the whole set of web gear. Weapon-specific web gear is too specific, in my opinion. Regards, – Doug Carlton