Do you need the National Dex to get swarm Pokemon in HG/SS? I want to get a Ralts, since I don't have wi-fi to trade one over, and I know it's a swarm Pokemon in HG/SS. Serebii says you get swarm Pokemon by listening the Mary and Oak radio thing, but all I get is Oak saying the location of some common Pokemon and Mary saying some random poop that really has nothing to do with the Pokemon (Or anything, really).
My guess is that you should get the National Dex first, or trade one to that version.