Renderlayer reflection problem

I have two questions that I hope folks can help with:

  1. I am trying to render an image as a linear workflow, separating parts of the image, but I am finding that reflections in part of the image are showing objects that are not on that part of the workflow. I want to be able to separate the reflection of a light from the reflection of the ‘glow’ round the light ( faked with an object) but I find that I am getting both reflected all the time. They are on separate layers and separate renderlayers, but I read that the octree and shadowbuffer passes are common to all, so they will still be seen even though they are on missing layers. Is there a wokraround for this other than rendering a separate image and compositing it back in?

  2. Does the mix node automatically handle zbuffering, or would I need to use the zcombine node to put a background and foreground together?