Rockwool vs. Soil.
Whichever floats your boat.

Clone adaptation to 250W MH @ 18/6...
It's a weed... it'll survive.

As to ferts...
It is just my opinion, but I wouldn't use Miracle Grow.
I know Garden Knowm (and others) have grown successfully (and very good) using Miracle Grow, so my suggestion is just an opinion.

Nutrients.... there's lots of products out there.
Yeap, depending on your mix of them, you're either veggin' or flowering.