is it posibble to add just to certain objects edge via the compositor?
The Kirsh filter works well. remember you are working in compositor without real knowledge of geometry, only Z and colors.
If you want to use the Edge Output option, create another fully linked scene, and turn Edge on in that Scene. Then, using Object ID you can mask from one RenderLayer the objects you want with an edge. OR, just delete the objects from the second scene that you do not want to be Edged, and Z-Combine them with the first RenderLayer.