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As you said planting straight into the final pot would have probably saved you the hassle of transplanting but now you learned for next time! One sure way to know your plant is ready to be transplanted is when the roots have started to grow out of the drain holes at the bottom of the pots. But if I needed to transplant, I'd transplant once the 2nd set of leaves starts growing which is just enough for the plant to grow a small but stable root system that can withstand the stress and to reduce the total amount of stress the plant suffers during the transition. The BioBizz "All-Mix" is a good option as potting soil because it has good aeration/drainage (perlite and coco mix) and has high nutrient absorbing capabilities (organic soil). Hope this helps! Good luck and happy growing :)

I would wait and see what happens.. Flushing soil is not simple as Just dumping water until a bunch comes out of the bottom. There is a whole living, breathing environment inside your soil and root zone. And things are alive and learning to thrive on the conditions they are growing in. You strip the soil and your plant will be much worse for it. Because the balance she is working so hard to achieve will be thrown off.