Does (a dragon’s) natural armour stack with barding-armour? [duplicate]

Logic suggests dragons may have interest in armour &/or barding, such as making draconic-scale armour from their cast-off scales. Smaug wore iron scales & hard gems after all. But how does this stack-up in 5e?

If i missed the StackExchange 5e answer, my apologies in advance / could not find it.

Two possibilities abound:

1. Of course they stack! Using Smaug from The Hobbit as cannon-canon, extra armour is always good. Example: add dragon’s value of 22 to barding-armour value of 8 or so for a total of… 30? Remove the dexterity bonuses (little dragon-joke here, all wyrms have horrid dexterity) – and throw away Barkskin, as that only works for trees.

2. Of course they don’t stack!! This is 5e and we work with sliding scale / curved averages / weighted probabilities or whatever. One (must?) find(s) the best of the two results. Just as a barbarian (constitution) or monk (wisdom) loses all AC bonuses by wearing a leather jacket on a cool day, so too must dragons struggle. We best not mention helmets. Or a gargantuan dragon with +2 to AC for a wooden buckler on one, um, finger. Ten wee bucklers for +20 AC?

But i digress. Let us ask The Question:

Would a dragon’s ‘extra’ armour-barding stack?