Not sure if this would work, since Im new to the
TES games. But the end perks in PIckpocketing
allow you to take equipped weapons and items.

Is it possible to stealth to an enemy, and pickpocket
there equipped weapon before you fight them?
(especially in dark dungeons).

possibly never seen anyone do it tho

I'd assume this is a good possibility.

Keep in mind the chances are still based on weight/value for the item, so even though you now have the ability to take equipped items, deftly plucking that Daedric war axe out of someone's hand will be... difficult at best. That said, it SHOULD be possible, just most people would rather bury a dagger between their ribs and take the axe off a dead body instead. Less hassle, and less chance of an epic mistake ruining your day.

Yes

From the looks of it you can pickpocket everything in their inventory and replace it with a bottle of poison. My first character is planning on being non-violent, but uses the calm effect on aggressive npcs and then steals all their stuff.

Also hoping that when I do this the NPC will realize that they have no means of hurting me and will just leave me alone or flee rather than charging at me with their fists up... we'll see soon, but im guessing theyll charge like stupid animals :/

True enough, but Im thinking it will be real good
for taking equipped items off high level NPCS
in late game (those you would not fight anyhow
like kings, lol).

I can't see any reason why you couldn't.