If I use poison that damages magicka or stamina against NPC's, will they eventually not be able to cast spells or use power attacks? Basically do NPC's in this game have a finite amount of magicka and/or stamina?

yes. they both work.

It does. They just usually have so much it doesn't make a difference. Especially against mages.

And power attacks can be done with a single point of stamina by player, if the same applies to NPCs the poisons are completely pointless

They should've made spells only need one minimum of magicka then if that's how it works.

they have a magicka cost amount for a reason.

So....what about melee power attacks or shield bashing? Don't they have stamina costs for a reason?

Stamina has a minimum of 1.
magicka has a set cost which can be decreased.

And I'm saying that discrepancy shouldn't exist. If you run out of stamina, you shouldn't be allowed to power attack until it recharges to a sufficient amount, since you can't cast spells with a minimum of one magicka.

Yes, mages in particular usually have high magicka and low HP so IMO you are just wasting your time if you go using magicka damage on them. Just hit them with a big stick and they die.

I'm playing a modded game and mages are pretty difficult that's why I was wondering if it was worth using the damage magicka poisons. Even against boss type mages with high HP it's still not worth it I'm guessing?

its not the same at all.