What type of soil can I use in the flats?
Any type at all will work. Mixes with soil for potting, or "soilless mixes" all work fine.
Avoid any soil mixes that say "weed-free" because they can contain an "anti-germinant". This is a chemical which is mixed into the soil to prevent any weed seeds the soil might contain from germinating. Unfortunately it also keeps ALL seeds from germinating.
What's MY favorite? I like the cheapest one I can find, lol! I do a LOT of Winter Sowing...it's fun and is very addictive. So I look for the biggest and largest bag of soil I can find. It's a very economical choice to get the biggest bag because it costs less per pound or quart. Plus, with more soil, I can sow more flats!