Velcro holds well when the forces on it are a shear type load and a 5-6 pound weight is not going to come loose unless your equipment and the soft pink body attached to it comes to a serious and sudden halt. And in that case you would have a lot more to worry about than about losing 4-6 lbs of weight. In any event, you could use either hard or soft weights.
My main objection to tank band mounted weights is that they present additional entanglement opportunities. If you are only looking to add 4-6 lbs, and are using an aluminum backplate plate, you could go to a stainless steel plate. If you already have a stainless steel plate you could add another 6 pounds by stacking another stainless plate.
If you go the XS pouch route I would consider adding them to the Waist strap of your BP harness instead and place them aft of the D-rings as close to the BP as possible.
However since you are only adding 4-6 pounds, you could also simply thread a 2-3 lb hard weight onto each side of the waist strap of your BP wing and secure them as close as possible to the backplate with a tri-glide keeper.
I have tried Halcyon's integrated weight pouches and I did not like them at all and eventually ended up using a set of Dive Rite weight pouches. They are quick releasable, keep the weight much closer to the backplate than the Halcyon pouches, and come in pairs in two sizes with either 16 or 32 lb capacity.
A sort of DIY alternative if you want weight on the tank is to get a cam band and then thread a regular hard weight on it and attach to the tank. If you do your own sewing and just buy the cam buckle and strap separately it can be done for around $5.00. It is very secure but again presents some potential line traps.