People veg with HPS all the time. Full spectrum and mixed can Always be done in both stages. It's just harder to flower with only blue spectrum. But we have all seen it done. People at the Dro store don't know shit, they like to think they do because they're all nerds who want to grow. But don't.
Those who Do, grow Pot, those who don't, work at the Dro store. LOL!
The plants will grow faster once they hit the res, the more light the better. Doesn't matter what spectrum