Well the Dark Angels are solid loyalists but try to avoid the attention of the Inquisition. I can see them easily allying wtih any of the Imperial factions with the definite exception of Grey Knights and the possible exception of the
SoB. I think they would even be fine with allying with space Wolves on the basis that they might have a formal competitive relationship, they don't actively avoid each other. You would of course need to fight a challenge between chamnpions from each part of your force before or after each game you play
In terms of Xenos, the Dark Angels are supposed to be practical with great tactical awareness, but I don't think they're particularly ultra-xenophobic, so Eldar or tau could probably be incorporated easily enough. I would suggest that Orks would be a step too far given that Belial got a serious drubbing at the hands of Ghazgull.
Obviously anything with even a hint of chaos would be right out