They won't need a light for very long if they are in the house, I have some that range in age between 5 days and 3 weeks that don't have any heat source. They also spend the entire day outside unless it is raining. I feed mine flock raiser almost exclusively the first couple weeks, after that they should have as much forage (grass) as possible. They are herbivores, eating mostly grasses. Mine only get a small amount of feed twice a day, other than that they have all the fresh clover they want
Cardboard probably won't last long, geese are messy! I would get a plastic rubbermaid tote to put them in
Also, I've found that a tall waterer that they can't tip over, as opposed to a chick-type waterer, seems to work better. Just be sure they can reach over the side
Mine have a margarine bowl with a hole cut in the lid for their water. It has a pretty good-sized rock in the bottom to help weigh it down. When they aren't quite so tall they have a plastic coffee can with holes cut in the sides, just big enough for them to get their heads in.
Good luck, goslings are very addictive!