I know, this is your second question part. As I said, I have same plans - also for inside rooms, caves etc just I’m working on other game parts now and not tried it. But I’m 99,99% sure this must be possible and easy, same way like without world composition. World composition just load landscape chunk and inside this chunk, everything (blueprints, actors (bp too…), triggers) works like on single landscape, map. I can’t believe this is not possible 
Moreover. On start do test like this - create simple World Composition map and this this simple streaming. Generally don’t start from creating landscape and environment first like a many people do, because this is bad way. First work on game code or on code and landscape same time, because this is all connected. Worse scenario, you will spend months on environment and you will have to redo everything from start. This happens.