I dont think you can use both numbering (roman and regular) in a Library.

For some series, i created a 'series index'
book title and set the series to seriesname[0]
(I just copy and paste from where ever into the book editor)
While custom columns fill the screen, and take a bit more processing, The (relational) DATABASE only gets entries for books that have data. Calibre is NOT a flatfile DB