I am always good for Stone to talk with somebody :D It's one of his favorite things. He'll also think Castiel is the same "species" as Azriaphale, cuz "angel" means nothing to an alien :B

The idea of Bucky actually teaching an angel is making me gleeful. Can he teach him how to fight without a weapon? Come join his classes/get outside class training? :D They could discover that during a spar, easily, Bucky's game for letting people whale on him for "training" and would absolutely point out where he's lacking when asked an opinion.