To select only the armour stands you have placed, use tags. Stand near the ten stands, and do:
tag @e[type=minecraft:armor_stand,limit=10,sort=nearest] add placedStand
This will add the placedStand tag to the ten nearest armour stands. placedStand can be whatever you want, it just needs to be consistent with the next selector (with tag=)
To select a random armour stand, @r no longer works for all entities iirc - even if you specify a type. However, @e can, if you set a limit of 1 (to select a single armour stand) and sort by random.
You should have something along these lines:
execute at @e[type=minecraft:armor_stand,limit=1,sort=random] run setblock ~ ~-2 ~ minecraft:redstone_block
~ ~-2 ~ will be two blocks below, with ~ ~ ~ corresponding to the armour stand's position and ~ ~-1 ~ the wool block below.