Get your HPS, and start using it right away.

Make sure there is ventilation in that area, even now. And make sure it is powerful enough to keep the temps down when you have a stronger light.

Go easy with any Miracle Grow products that you use, especially ones formulated for use on rose gardens and outdoor flowers. Yes, it's a fertilizer and works like a fertilizer should work, but it's probably a complete (and potent) fertilizer package, and can be damaging to other plants with sensitive needs.

Think about looking into organic fertilizers, which are generally much more forgiving in soil. Fish emulsion is a great way to supplement nitrogen, as are many guanos, and other natural products. In bloom, you'll have all sorts of possibilities to choose from. Keep the MG in case you need to treat with a complete fertilizer later on down the road.