Check the z coordinate with the whereami armband. The editor sets it at 0 per default and you have to change it manually. I was looking for my placeables for hours the other day until I checked the z coordinate
However, you don't have to worry for creatures: gravity does it's job and they will "fall" on the floor by default
...and yes, you can put creatures on top of placeables as long as the placeables cannot be walked through. Edit: ah no. The examples I had in mind were already attached to the module. It depends on the walkmesh.