2-е неправильно
Sally's hair is longer than Nelly's.
Для 6-го несколько вариантов может быть, смотря что автор хотел сравнить: девочек, юбки...:
This skirt is as beautiful as that one.
My skirt is as as beautiful as hers.
The girl with long hair is as beautiful as the girl with short hair.
This girl is as beautiful as that one.