I don't think you need the "medicated" chick starter any more, but I'd probably still give them some kind of "growth" food, like Ariel301 mentioned, with a little extra protein. I've always been told that scratch was a treat, not a staple. Mine love it too, but I only throw a couple handfuls out for them in the afternoon. If they free range, and get lots of bugs, and fresh plants, then you might be just fine. But if it were me, I'd still offer them non-medicated chick food for another 2 months, then switch to layer.
Congrats and Best of luck with your new flock!! They are so much fun!