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Once the seedling has popped up and is beginning to shed it's seed shell i'd transplant it. I wouldn't let it sit long, if the tap root grows through the bottom you should transplant immediately.
As for soil you have 80 bajillion choices. The most popular is foxfarms line, pro-mix BP is a good mixture as well, hell you could even go and buy some nice soil from a nursery provided it's nice and airy and retains moisture well.