For your soil, are you using any perlite? If so, that could be why the roots are growing so fast... Last year, my two week old seedling had roots coming out of an 8" pot, and they were growing more everyday... It's not a bad thing, unless you don't have bigger pots to transplant in, but root growth is important, as I'm sure you know.
So far, nice plants. When are you going to start feeding them, and with what?