In Japanese and other languages, the word for “of course” carries the meaning of “naturally” or “surely,” and so by extension the Japanese word “Mochiron” is often used with a meaning of consent or permission.
The word for 'no' in Japanese is いいえ (iie) or the more familiar いや (iya). But to say or hear 'no' is generally uncomfortable for the Japanese.
As a question, そう sou can be used by itself with a rising tone, or followed by か ka or ですか desu ka. It means "that is right," or "that is so," and is used as an affirmative answer to a question.
~KEKKOU DESU~ does sound more formal. The one thing about choosing which one to use, especially when you are new at Japanese is that when you use ~KEKKOU DESU~ there is no mistake in what you mean but with ~II DESU~ you might mean to say "No Thank You" and actually say "That would be good".