Late Roman Names / Practices

[Robert Vermaat]

I'm shooting for the 5th century, but Flavius Aetius is out ( I can't use a historic character.) Would Flavius also be rendered as flavianus ?
Would Valerius Aetius be a possibility ?

I'm afraid that you're stuck with Flavius as a name. If you want to be named differently, it comes to what's after Flavius (I'm Flavius Valerius btw, happy to make your acquaintance).

If you follow this link to Google Books you'll find a list of people named Flavius.
Aetius' father was Flavius Gaudentius, and it appears that there was another general (in the East) also named Aetius.
Scroll up to page 20 to find more on other people named Aetius.