Including recently opened account or accounts that have barely made any posts?
Yes.
I don't see how a newbie would be aware of a competition like this in an hidden board.
Competitions & Contest board isn't hidden, you can even view it while not signed-in.
This is an art contest, rank and other tight requirements shouldn't hinder someone in showing their art skills.
I'm applying for atleast two section, should I post them together or I can post them separately?
You can separate them or post them together, there's no specific instructions for that. As long as you've posted your entry in this thread, that'll be fine.