I've never seen any evidence that these murders were anything but a literary invention of Colleen McCullough's. It's been a long time since I read "The First Man in Rome" but I think she addresses this in the historical notes at at the end of the book.

See for example http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0104%3Aalphabetic+letter%3DM%3Aentry+group%3D23%3Aentry%3Dmetellus-bio-13

Cicero accused Quintus Varrius of poisoning Metellus Numidicus, but that account is viewed as very dubious by historians.