Looking good. Is your ballast compatible with MH bulbs? You can definitely veg with HPS however you will have taller plants and longer node spacing. With an MH bulb, you get tighter node spacing and better lateral branching. It's also depends on the strain and your growing techniques. Some dudes swear by hps start to finish. I normally use MH till the 3rd week of flower. Keeps the stretch down as I'm limited in vertical space.
As for distance, 18-24 inches is about right in veg. Again, strain dependant, some will handle the higher leaf surface temperatures, some will not. For flower, 12-18 inches. Lots of trial and error. Maintaining the distance is also important. Check weekly and adjust.
Keep up the good work!
