hello. i wanna ask if i buy Harvey Walters Investigator Starter Deck can i use its cards to form a deck from the core set cards for Daisy Walker: The Librarian ?

Yes, I think all the player cards (except maybe Barkham Horror cards) are compatible and are available to any investigator who can use them based on that investigator's deck building requirements.

Yes. Only Barkham player and Investigator cards are specific to that scenario.

Keeping the investigator decks separate or including the cards for general play is up to you. The only thing we noticed it that the random basic weaknesses created for the pre built investigators are pretty geared to that investigator. Some of those weaknesses would be a non issue to other investigators. So we don’t use them in the general pool and if we play one of them, we give them the one they came with. That is just us though.