Books aren't evidence. IRL, even if I met a
Septimus who literally did have six elder brothers, I'd still think it showed a failure of imagination by the parents. And if I met a
Septimus who was an only child, say, I'd regret the parents' choice. It sounds good, and of course
Sextus would be impossible -
Tertius happens, also
Quintus and
Quentin, possibly even
Octavius though usually without the number meaning - but it's long enough to need a nn: Sep would be too close to
Sepp (like the nasty Herr Blatter) and Seb, so there's a likelihood of confusion.
Thinking about it, I'd prefer
Sebastian. Nicer name; same length; similar vibe.