Matilda - nice, fairly well used, I don't like Matty for short though, which I'd worry about
Cordelia - eww, yuk, very old fashioned, Uber posh
Cecily - Nope, similar to Cordelia, Cecilia is a lot nicer
Romilly - still 'old/vintage posh' but I've always had a soft spot for this name ☺️
Magnus - Naff
Clarence - The angel from It's a Wonderful Life "Attaboy, Clarence!" Very old fashioned, rather posh
Emil - ewww
Frederick - I like Fred
Gustav - weird unless you have Scandinavian connections.
If you're going for circa 1920's posh, I think you've nailed it, but they are not my cup of tea.
Good luck!