Howdy
Let me help you out here because I carry in a shoulder holster a lot and I have for years.
One of THE best shoulder holsters ever made is the Alessi rig designed by Louis Alessi. A master leather craftsman.
His holster is the finest leather and does not have a traditional retention strap.
His design features a Pull~Through one way snap that snaps over the front of the trigger guard. All you do is tug the firearm and you own it. It's lightning fast to draw from.
Your Galco is OK for now but, once you are absolutely certain which exact firearm will be your every day carry then go for the Alessi. You can even keep your Galco harness system and just go for the Alessi holster. I would try to select a harness system with a soft wide shoulder strap though for better all day comfort.
Also you might find yourself with an sore shoulder for a couple of weeks until your shoulder gets used to the weight of a firearm pulling down on it for 8, 10, or 12 hours. You do get used to it though.
If you are not overweight then you should adjust your shoulder holster so that it is up just below your armpit with the muzzle canted slightly upward. Do not wear it down low near your belt. That is for TV only.
Some folks who are very overweight have trouble drawing from under the opposing armpit and a shoulder rig might not be suitable for them.
The most important thing to remember with Horizontal Shoulder Carry is to carry a high quality firearm in a good holster and basically leave it alone until you need it.
If you ever DO need to re-holster after a defensive scenario then just make sure that you re-holster by turning your body so that your muzzle is (of course) pointed in a known safe direction or remove your shoulder rig - then re-holster and put it back on.
Practice your draw at home by following all Firearm CLEAR rules & with practice with an Alessi rig you'll find that you can get your firearm on target as quickly as a draw from the waist.
I am familiar with the McCann Crucible Series and Kelly is a fantastic instructor. His KYDEX rig right at 3:00 is great for impressive range work but, you obviously already know that it will not work too well for you out on the street. You don't want to walk around with a big bump on your strong side hip.
You can develop EXACTLY the same easy, fluid motion no matter where your firearm is located.
Practice will do it for you.
FAST/Speed to the firearm...then a proper grip...then finger off the trigger & out of the trigger guard since for the fastest draw you'll be lifting your support arm so that you're not covering your support arm as part of your draw sequence & then out to the target exactly al la McCann. It's hard to explain but you'll get the hang of it quickly.
I wish I owned a video camera but, sadly I don't or I would do a vid of the draw.
Just practice.
Oh here is one link that provides a little more information on the Alessi BodyGuard
...CLICK HERE