Do I use graphite paint over a sterling silver plated pendant? — Maker Monologues
You want to apply your conductive paint, whether that be graphite or copper paint, anywhere you want the copper to “form” over. You do not need to apply a conductive paint over any surface that is already conductive, which means any surface that is already metal.
“There are no non-conductive metals; all metals conduct electricity, though some do this more effectively than others. Thanks to the molecular structure of metals, they are all relatively effective conductors, though silver is the most conductive metal known to man.” – Reference.com
So you don’t need to apply conductive paint over the sterling silver-plated pendant. Therefore, yes, just apply the conductive paint over the epoxy.
For your question about using broken pieces in copper electroforming, you can absolutely use a broken piece of jewelry in electroforming. You can really do anything you can imagine with this craft! =)