very easy to load Blender objects (scenes, textures, whatever!) from one Blender file to another. Go to file, then there is “Append”. You can either link them in (when you do that, you can not edit the objects from the big file, but when you go and edit the objects in the little files, they will update in the big file automatically) or append them, which will load all of the data into the big file and they will be fully editable. If you append (NOT link) for the purpose of comining little files, you can delete the little files when done.
You second one…sorry you can’t do that with blender yet. You may be able to tweak the force and damp settings to make the particles stop at the surface, but they won’t bounce. However, look into Dynamica. This plugin will offer the capablity, but is not out yet.