Hi, happy new year from Spain
Any answer to my question?
Is the class decks expansion playable in the big box expansions, or only in the core set?
Yes and no. It depends which box expansion you will be using. The no's would be Midnight and Mists of Zanaga. Midnight because that is actually addressed in the rules. In MoZ you and the other players will need to work together and not against each other or the game may win.
Yes to Sands but keep in mind that the encounters in that expansion have day and night health point differences. I don't see any problems with Frozen Wastes or Isle of Dread.
Just keep in mind that all players should agree to use the Character decks in which ever expansion you may choose. I hope this helps.
Bill