Slitherine

I am just starting up on multiplayer. I played a few hours on the single player base campaign and the US Pacific campaign. I have the DLCs for the US Pacific, Morning Sun, Rising Sun and Marines.

Are the multiplayer scenarios separate from the DLCs? Do you get extra multiplayer scenarios for each DLC?

Can I join a multiplayer scenario if someone else hosts using a DLC I don't have?

Multiplayer scenarios are designed specifically for 2-4 humans/AI combinations. They are usually quite different battles than the campaign scenarios. A completly different experience IMO, highly recommended.
You only need ownership of at least one DLC to be able to play MP (or use the editor).
Since I own all DLCs I can't answer the rest of your questions. Others will hopefully chime in.

HI,

The Multiplayer scenarios are linked to the base game. As Erik says, you just need one (any) DLC purchase to enable all of the MP scenarios for play. It doesn't matter if you and your opponent have different DLCs or not.

If the other player has downloaded a custom scenario and hosted a game, you need to go and find that scenario and put it in your Documents/My Games/OOB/Scenarios folder. Check out the sticky posts in the scenario design sub-folder for a great collection.

(many by Erik ;0) )

Thanks