The easiest way to do what you ask is to simply use an environment map or a reflection texture. This does not require the compositor. Using an environment map doesn’t require raytracing and is faster, however an environment map is not as accurate as raytraced reflection.
If you insist on using raytraced reflections and the compositor your problem can be solved as wel, it only takes more work.
Your problem with the reflected objects is related to the SceneLayers, in your scene layers you enable the layers that should be rendered. The second pair of layers you can set, only indicate what layers of the scene need to separated, these layers are the ones with the dots. This might sound a bit confusing, but it basically means that even though an object is in a layers that is turned off in the 2nd pair of layer controls (the ones with the dot in them) as long as the layer is turned on in the 1st pair of layer controls it can still cast a shadow or a reflection or whatever in the resulting render.
To solve this you need 2 scenes with different SceneLayer settings, one with the “reflected objects” layer on for the armor and one with the “reflected objects” layer off for the floor. Then you can comp the armor from the first scene with the floor from the second. The only downside from this approach is that the reflected armor in the floor will not show its own reflections resulted from the reflected objects.