Nothing wrong with mixing tires--mountain bike racers do it all the time--but it's theoretically a good idea to put the larger tire in front. If one end of the bike's going to slide, you want it to be the back. In practice, though, I doubt you'll notice the difference.
FWIW, there's a school of thought that nobody who weighs more than about 110 pounds should use tires that skinny. I've ridden 21s (I weigh 230), and it was awful.