Most sellers of indoor "bonsai starters" will sell them growing in plain old potting soil in 2" or 3" pots. So, there's no harm in growing it in potting soil. However, you do have to be aware that once you put it into a shallower (bonsai) pot, "training" or otherwise, you'll need to switch over. Because the composition of the soil is so different, the tree may experience shock moving over. I personally have found that it's easier to do this as soon as possible, waiting no more than half a year, to do so. Smaller, younger trees are going to be more resilient than older, more established trees, especially trees that are already being trained, when it comes to such vast changes in growing conditions. I also prefer a more free-draining mix for succulents anyway, and, at most, even if growing as a housplant, I would use no more than 50% potting soil to sand (or other aggregate).