For shoulders, I like the DGX trim pockets as they hold enough for trim, 3 lb. (edit: or 4), are not bulky, and have no corner holes to lose small weights. Below are three pockets laid out next to my plate.
XS Scuba, left, holds ~5 lb. Corners are open. Bit big on shoulder. Shown holding 3lb hard + 1 lb
DGX, mid, holds ~3 lb. No holes, good if have sub 1lb. weights. Velcro is generous. Shown with 3lb. hard. loose but full fit, also holds 2 1 lb. bullets plus a long 1/2 pound weight, or small sub pound weights securely.
Scubapro, right, hold ~4lb. No holes, seals tight plus buckle whose strap usually loosens. Shown with 2 1lb. bullets. It is sitting on top of a 2lb. weight from the weight belt. My pool weight belt is two of these secured as below.
Plate shoulder shows where I have the DGX pockets on my other plate. Currently I have bungie loops tightening up their straps around the shoulder strap. It is relatively firm but moveable hold, but I'm still testing it.
For really not moving, I melt holes in the webbing, with pliers, nail, and stove burner as you can see on the waist belt, and tie pocket straps to the 2" webbing. For super paranoia, two sets of them on each side of 2" webbing midline, though I only did one set in the above picture. Ideally I would like tracks of such holes down my webbing for securing things, I'm not enamored of 2" metal clips that dig into my sides. Not an issue on the rig waist belt, but on weight belts and shoulder straps it's a pain.
Edit: Soft weights that can fit:
XS Scuba: 5 lb. (1 lb. + 4 lb.) (listed for 5 lb. bagged or hard) You might cram 6 lb. in at the expense of the velcro, and the 1 lb. I used is preventing full use of the pocket bottom.
ScubaPro: 4 lb. (listed for 1kg or 2.2 lb.)
DGX: 4 lb. crammed a bit past full. (listed for 4lb. hard) The DGX would mount on that belt turned 90 degrees.