No problem with the pots not being full. You can just add more dirt. They look kinda stretchy any way, so more dirt will help keep them stable and upright.
The light does seem close for seedlings, try 2ft away for now. When you get a chance, buy an exhaust fan and hook it up to your hood asap. Miraclegro is not the best to grow in, but you can still get some good smoke from it. I remember it being very acidic so some lime will help out.
Lastly, when it comes to watering, use the lift method. Lift the pots to see how heavy they are. When they feel light, add water. Dont water if the pots feels heavy or you'll end up over watering.