Do stances have any special rules I should know about... I don't see any in the rules.

ALSO

Multiple attacks, never go into the momentum? I have this one card, that makes all attacks go into my momentum... does it also not work?

Stances? There should be no extra rules with stances.
And no face down cards and multiples can never go into your momentum so they would be discarded

forgot to say, multiple cards just become a face down card after they resolve. that is why they cannot be added to your momentum

Incorrect.

Facedown attacks CAN go to momentum if they aren't multiple copies. For example, using Twelve's 1/5 foundation (with all the numbers) to copy your opponent's attack with a facedown card from your hand - if your facedown copy of your opponent's attack deals damage, it will be eligible to go to momentum during the end phase like any attack.

The text of the Multiple enhance in the TR specifically states "any multiple copy that would go to momentum is discarded instead". So if you have a card that says "Add all attacks in your card pool to your momentum" and you have multiple copies in your card pool, they will be discarded instead of going to your momentum.

Stance is just a keyword like Punch, Kick, Ranged... doesn't actually do anything. To my knowledge nothing in the game makes any reference to the Stance keyword.