The developers were really thoughtful when it comes to making missions. In the Mission Editor, you don't need any of the modules to make other missions. As an example, I don't need the BoBp module to make a mission that starts with BoBp planes. However, you won't be able to play (or test) that mission unless you have the correct module - in this case, BoBp. You can use ALL of the assets in the mission editor for you mission - and while us veterans may complain that there's not enough options; there really is a lot. So go wild, make cool missions and use any entity you see fit.
Even better, the maps are only tied to the campaign (and quick mission builder I believe). This was not the case originally. Meaning, you can use any map to make a mission, and as long as the user has the PLANE/TANK module used to fly in the mission, he can play that map. This is why anyone in multiplayer can fly with other pilots - most mission designers will include at least one plane from each of the modules for the BoX series - and everyone can play even if they don't own the module/map.