I think you will be fine using this product. I would not put the powder directly in your peat/perlite .
My understanding of the product is that you add it to your regular watering nutrient mix at a rate of .5 to 1.0 ml per liter of nutrient solution. If you want you could water the pots with a light nutrient solution and about .5 ml per liter of Root-Stim prior to putting the plants in the pot.
I use a similar product with each watering for the first 4 weeks. Really helps build some nice roots.