"Hers" is only used alone. With another word, use "her."
When is הם/הן etc needed to for specifying the subject, and when can it be dropped?
אלה הם חדרי השנה שלי ושלה
אלה חדרי השנה שלי ושלה
I'm not sure if there are strict rules. In this case both options are OK.
A useful rule of thumb is to add the copulas when the sentence is similar to "<noun> is <noun>", and not to add when it's "<noun> is <adjective>".