Don't use tinfoil. It's not that reflective, but it can put little pinpoints of ultra-bright light on the plant and burn the leaves. Make sure you are aware of flammability. Cardboard can catch fire and even cfl fixtures can build up heat, although probably not enough to set the box on fire. But anything's possible. Headshake knows a lot about using cardboard so try reading his grow log. It's also very informative in general not just related to cardboard boxes.

I would recommend holding off on nutrients until you start to see the leaves turning a bit yellow. Young plants (and grown ones) burn very easily from nutes. Once the plants are growing I wouldn't recommend trying to get the soil off their roots. Have you ever grown container plants before? You should just take a bigger pot, put enough soil that the height of soil in your cup will reach the top of the new pot, water that soil, turn the cup upside down and tap it with the stem of the plant between two fingers of the other hand so you won't drop it. Then put the soil and root ball into your new pot, put enough soil to fill in the edges and a little more on top, then water it and you're done. You want to disturb the roots as little as possible.