Quoted:
1) Is there any reason NOT to put the hydration carier in the center of the rear of the carrier?
It makes sitting in a vehicle a pain.
I've always like to be able to mix and match kit for my needs at a given time. FWIW, I spent four years on active duty with the USMC in the 90s, and after 9/11 I spent more than four years in Afghanistan as a security contractor for the U.S. Gov't. For a few trips I used a plate hanger with everything on it, or a Paraclete RAV, but I quickly found myself moving towards slick armor (no MOLLE at all) with separate chest rig, belt/suspenders, and Eagle Yote for my water. The various layers meant I didn't have to put on 70 pounds of GI Joe Gear just to go to the range. Or, whenever we got indirect fire (very frequently, I was only about 10KM from Pakistan the last couple of years I was there) I could just grab body armor, helmet, radios, and binoculars before I went to my spot. I'd take my rifle too (I kept two mags mounted as my ready to go load) but my job was really about observation/communication/coordination by this point, not engaging Timmy with my rifle.
All that said, if you are contemplating a rig for serious use (meaning you need to kill someone so they don't kill you or your family) train with it
a lot, in uncomfortable positions, with distractions, etc. Pick a difficult course of fire, but before you do it, do fifty push ups with all your gear before you shoot. If you're shooting at night, have someone double flash you in the eyes with white light a moment before the whistle blows for you to go hot. Training is what makes you competent, not gear. Too many people seem to forget that, and seem to think that if they dress like a door kicker and have gear like a door kicker, well, they're a door kicker.
NO. When shit goes sideways, you don't rise to the occasion. You fall to your level of training and practice.
If this is just for range use, I'd skip any sort of rig altogether and just get a nice rifle bag for my AR, ammo, and magazines. If you plan to take some classes that involve moving, tactical shooting, etc. there are any number of economical chest rigs out there that will hold 4-6 rifle mags, a couple of pistol mags, accessories, etc.