what is the difference between فتاة and بنت in their meaning as girl? Are they interchangeable or does one have special conditions the other doesn't؟
To be precise, they are different, But people today (specially in dialect forms and street lang) do use (bint) more. The word (bint) in classical Arabic is more associated to (daughter) rather than just (girl). On the other hand, (فتاة) [fatát] is more associated with (girl), a young girl to be specific. And its masculine counterpart is (فتى) [fatá] - young boy.