I think you're mixing things up. Layouts are one thing, Event sheets are another. In layout you'll have objects (sprites, texts, etc), while in event sheets you'll have code (events, actions, conditions, etc).
The assets layout is there just to guarantee that anything you'll need to create dynamically is available, since Construct requires that at least one instance of an object exists in order to create it dynamically (by using the "Create object" or "Spawn another object" events). This layout has no event sheet associated to it. I'm not sure this is the most efficient way and I can't tell if all objects are loaded or only the ones you create (I think it's the second option).
You can also have separate event sheets with code you'll reuse and just include then at the layouts for each level. But I'm not sure having enemy specific event sheets that you include only in layouts that have those enemies is worth it. In my understanding, if you have code that is never triggered it just won't do a thing. Of course it WILL be loaded, but, again, I don't believe this will affect significantly the loading times and cpu use.
Maybe Ashley could help us all on this...