Don't veg under an HPS!!! It wont hurt the plants, but the spectrum of light is completely different than what is needed/desired. I would recommend fluoros, a 4 foot t5 preferably. If you want, switch them to a metal halide after 2 weeks or so under the fluoros. But i definitely don't like the reaction seedlings give to metal halides, which is usually shock or extreme stretching, due to how far the light has to be away. Get a nice t5 setup and keep it around 4 inches away from your seedling's tops, up until 2 weeks. Beyond that, look into a Metal Halide, or go ahead and flower them. But once again, an HPS is rather useless to raise plants in the vegetative stage. Hope this helps.