The Eurosceptic parties (Red Greens, LA and DPP) + the People's Movement against the EU have formed an alliance and agreed on setting aside their differences and presenting a common "No" platform.
Wow. I'm amazed that even Enhedslisten considers incidental cooperation with DF acceptable.
Well if they should choose a party not to cooperate with, I think LA would be the more obvious choice. But to your entire question, Danish politics right now don't have any untouchable parties, all parties can make deals with each others, and Red-Greens have made deal with both LA and DPP before. While I'm not a fan of the Red-Greens, they're a lot more practical and pragmatic than we sometimes gives them credit for, the fact alone that they could unite a bunch of far-left fractions which hated each other show how pragmatic they are.