Dual channel is almost always the preferred method of installing ram (2x8). One 16gb RAM stick will perform much less efficiently than 2 8GB RAM sticks.
For your second question, mixing RAM of different sizes and clock speeds generally doesn't work. Even RAM that is exactly the same, but bought at two different times, may not work. They're put into kits because they're made to work together, and that may not always be the case when you mix RAM.