Well, creating battlemaps in GIMP is pretty much all I do here... And I get plenty of great advices from talented posters like Jacktannery.
Unfortunately, I am on the phone which hardly makes justice to a map, soI can't really comment.
But to answer your question, I drop my furniture in the map, then fuse this layer down to a new one, when I can move and resize the objects as needed. Once I am satisfied, I fuse this new layerdown to another one so that all my furniture end up on one or two layers. I don't like to multiply layers unnecessarily, on the other hand it is good when piling up objects and working with shadows (the chair should cast a shadow on the carpet but not on the table....).
As an aside, Jack game me a great advice to manage lighting source (it's in the latest project I worked on, the Yuan Ti temple). That should work great for a subterranean scene like the one you are working on..