Not a series, but a subject

I don't think
http://www.librarything.com/series/Roman+Imperial+Biographies
is a real series. The publishers differ, for one thing, and not in the way that one publisher publishes it in the UK and another in the US.

It looks like you have several people that disagree with you.

Well it is a publisher series at the very least, since Routledge has issued these with the series title"Roman Imperial Biographies": https://www.routledge.com/Roman-Imperial-Biographies/book-series/SE0265

I don't know enough about the books to say whether they can be considered a regular series (i.e., whether the books were written specifically for the series or whether it also includes titles originally published elsewhere).

It sure looks like a Publisher Series to me.

OK, it seems I posted this observation twice. If anything, it's a publisher's series. I read many of the original editions of the books and those editions didn't say they were part of a series. So, no, they weren't written specifically for or as a series.