Ultimately, when rolling to set up an Aspect to take advantage of a defect or weak spot in an opponent's armor, you aren't necessarily making a Declaration/Assessment that it exists (though you are, somewhat) so much as giving it narrative importance as a source of plot advantage.
And no, in the RAW, "Armor" (the abstract mechanism, usually provided by physical armor but also by other things depending on context, which reduces a successful attack by X shifts of stress) does not stack with other sources of "Armor" (the abstract mechanism, usually provided by physical armor but also by other things depending on context, which reduces a successful attack by X shifts of stress).