There is a wiki page that answers some of your questions https://github.com/AtomicGameEngine/AtomicGameEngine/wiki/Changing-how-models-are-imported . Drag and drop to the Editor does not work for Linux at the moment, the work around is to drag and drop the files in the OS. On Win and Mac both Drang and drop methods work.
Most assets types can be moved into the project in this way, the 3d model formats it will convert are : fbx, obj, blend, x, dae, 3ds, ms3d
Currently, the assimp library is used to convert the major model 3d formats, but it is not very good about reporting if the conversion worked or not. It can do FBX, as some of the examples demonstrate, but my experience has been it does not like older FBX and will silently fail, I now use the free (win,mac) Autodesk FBX converter to get a verison that assimp will successfully convert until assimp can do the job without it.
Other ways of getting models into AGE, is by using the Urho3D AssetImporter, which is a command line utility, so very hands on. And there is a Blender exporter https://github.com/AtomicGameEngine/AtomicGameEngine/wiki/Blender-to-Atomic-Game-Engine-Mesh-Exporter , which is a little less hands on.