I don't think Miracle Grow is big problem with seedlings. Its a weed for cripes sake and if a weed can't germinate in it what kind of seeds would grow in it? That said, I hate MG, Scotts, Monsanto like POISON! They are the fucking devil. Personally, I avoid their products. I would continue to use soil rather than rock wool to start if you are going to grow in soil. Buy a bag of seed starter mix (has more fine peat moss).
Are you keeping the lamps close enough to the plants ( like, not two feet, .6 meter away)