Starting in used soil shouldnt be an issue unless you used a shit ton of liquid nutrients meaning salts. Starter soil doesn't require much, seedlings get everything from their seed case needed for up to 2 weeks. Only thing they need early is a good environment, small amounts of water, and to be left alone. I use a weak light to sprout so mine generally stay under a dome and wont get watered until like day 2 or 3 from sprout then just a table spoon of water. If I don't use a dome and stronger light may need a tablespoon daily. Once I have to water a tbls 2ce a day I'll water 2tbls so on and so forth until I'm watering pot to slight runoff daily then it's time for transplant.