This Molle stuff is great (if you don't mind the weight penalty of rugged stuff) but it's all designed for attachment to horizontal straps. The shoulder straps of rucksacks are, in principle, good places for small pouches but, unless you wear them sideways, Molle pouches require that horizontal straps (perhaps those little Velcro ones) be wrapped around the shoulder straps first. Then you've got to stop those slipping down, so that means clamps or pins or some hard sewing. It's all a bit of a pain!
Most shoulder straps have a small horizontal strap. I think it's meant for a bladder hose? Not sure
