I prefer girl names for girls and boy names for boys, but in a nickname, it's okay, I guess.
I know a
Samantha who goes by
Sam a lot, and it doesn't seem weird. I think
Sam is more unisex.
I don't really care for
Ben and
Ron on girls though. I could see
Ron more than
Ben, probably because there are more girl names that have
Ron in them than girl names that have
Ben in them.