You don't wan to use air stones in SW tanks. One, they will cause your pH to be irradic and two, the bubbles bursting at the waters surface will cause a ton of salt creep around your tank.
As for the skimmers, if you're going to run one I'd go with the Hydor Slim-skim or the Tunze 9002. They are both around or slightly over $100 but worth the money. I would suggest not wasting your money on airstone skimmers (like the JBJ or Biocube ones) because IME they're just not worth the money. And if you're not wanting to spend $100 for a skimmer then just don't get one. It's not needed to run a skimmer on nano tanks, live rock and water changes are more then enough filtration for them. I ran my 28g like that for almost 2 years without any issues.