"Those women are famous."
does 女の人 have more of a sense of singularness than most nouns in japanese, making たち necessary here? or could you leave off the tachi and still be referring to more than one woman?
I was a little confused by that as well; saying 女の人たち out loud (or 女の子たち since I've seen that here too) and it doesn't feel right.
Is the の人 after 女 absolutely required? Is onna no hito a more formal form of onna?
Yep, 〜ら and 〜たち both mark the plural on some "people words", but 〜ら is usually more rough/less formal
I've seen 彼ら, but I've never seen 彼女ら. It's usually 彼女たち. If that's what you mean, then yeah. It's meant to be a forced pluralization of the noun.