When I have grown from seed the first leaves almost always yellow and fall off. Don't worry unless the true leaves yellow. As far as repotting, just look for the roots to come through the bottom of your paper pots (or tear a little hole to see how they are doing). If they look crowded before your ready to plant out then move up a size. I grew in my cool basement and found that a little heat from the bottom helps. They make special plant heating pads for that but if you grow in a warm spot they should be fine.