Working in Blender 2.79 using the Rigify Cat(Meta_Rig) I discovered problems with the front feet when trying to apply Automatic Weight Painting. After much tweeking and retopoing of the area Blender still didn't like it so I separated the fron feet from the main mesh, Auto Weighted the mesh to the rig, then reattached and hand weighted to offending area. Checked everything was working smoothly by moving the bones around, and finally highlighted the metarig and hit GENERATE RIG to create Widgit Rig. This took several tries and both Blender and computer reboots before it cleanly generated. But now it is there. Problem: The weighting does not seem to have carried over to the widgets as they are not controlling the mesh. How do I link the widgets to the weighted mesh? I asked a similar question back in June before working out the weight painting issues of the front feet. The discussion went into Chat because of it's length but has been erased because of time.
With Rigify, you need to generate the rig from the meta rig first, before weight painting.
In blender all control widgets etc are also bones, but may or may not be allowed to move the mesh, depending on whether the deform flag is set on them or not (this can be found in the bone properties tab in the properties editor). In the generated rigify rigs, the deform bones are hidden by default, and normally on the third to last layer: These are the ones you need to weight paint.
You may also find the following (slightly old) link useful: https://blenderartists.org/t/rigging-with-rigify/673045/2