NPK ratios are not necessarily what will burn your plants, the concentration of the nutrients overall, and usually too much Nitrogen. But a high nitrogen to potassium and phosphorous ratio is needed during veg, in other words a lot of nitrogen is required during aggressive vegetative growth.
Most pre-made/mixed soils will not be harmful or have any of the ingredients at high enough ratios to burn your plants. Using a seedling mix is even safer. After a healthy root system has developed, as indicated by a few sets of ‘true’ leaves, you can start adding a nutrient mix to your regular watering. During the vegetative growth season you’ll want a mix that is higher in nitrogen and during the flowering season you’ll want higher potassium and phosphorous.
Flushing actual “soil” does not make the soil become 0-0-0.