I love
Octavia, I think it would be darling on a little girl and very lovely and elegant on a young woman. It's near the top of my favourite's list :)
I don't think it matters at all about not being an eigth child, it's formal sounding but i don't feel its pretentious, just classy.
I love
Octavia Harriet and
Octavia Phoebe
btw
Octavia and
Violet would be an amazing sibset