Pardon my asking, but is it such a stretch to think that the Doctor has met with the Face of Boe numerous times beyond the tv series scope? The guy's more than 900 years old, and has traveled to many worlds and different points in time. We know he's been friends with Da Vinci, Puccini, Rembrandt, and many others. But have we ever seen any of these historically famous people on the show? No. He merely mentions them in passing as past acquaintances in his long life, a life that, for better of for worse, is never shown in full onscreen. So how is it improbable that he and the Face of Boe were buddies from a time and place long before? There's so much of the Doctor's past life that we know nothing about, but then that's also the beauty of it. New writers can come in and add new material to better enrich the universe and enchance our enjoyment of it.