The actors were in a movie called Ex Machina (which I may have spelled wrong) together, and had a lot of chemistry both on film and especially during the press tour. It probably comes at least partly from that. (Like how there were lots of Bane/Blake fics for The Dark Knight Rising, despite the fact that the characters don't share any screentime, because of the actors' previous popularity together in Inception.)
Re: Hux/Poe pairing?