I prefer
Rosemary to both
Rose and
Romy. As you said, they can be used as nns and it's also a good name in itself. Versatility is important.
I know a 24-year-old
Rosemary who doesn't go by a nn; she's always been happy with her name and never had a moment's trouble. So, yes, why not?
If I had to choose a
Rose name, I'd have difficulty choosing between
Rosemary and
Rosamond.