Your seedlings are going to stretch if they don't have enough light. When you introduce the blurp light, they might get stressed because of the increase in intensity and change in spectrum. I would go ahead and put the blurp up, but closely monitor for any negative reaction. Start the blurp out with at least 24" to 30" height above the canopy. If they like it after the first day, then inch it down little by little each day until you find the amount of light they will tolerate from it.