Maybe a stupid question, but I havent' found an answer yet: Is it allowed to play Order vs. Order or Destruction vs. Destruction?

I mean, if it is possible for me, playing dwarves f.e. to fight a friend of mine, playing dwarves too?

Yes, you are allowed to do that.

Yep. The only Order vs. Destruction thing is that you are not allowed Order cards and Destruction cards in the same deck.