Promoix is great. I've been using it for a couple years. I prefer the HP as apposed to the BX, the only difference being, HP (high porosity) has more perlite. Promix is not soil, it is a "soiless" medium, meaning, it lacks nutrients. Therefore, you will have complete control of all food given. There is enough food in the seed itself to feed up to 2 weeks. I start a light feed, 1/4 recommended dose @ day 10, increasing gradually until full dosage, around day 35-40, depending how she responds.
I'm not too familiar with HPS but I would think you may wanna raise the light a bit more. Let the stretch happen and lower accordingly. I find it's always better to err on the side of, too high, rather than too low.
I'll tag along and would be happy to help, if needed.
Good luck!!!