I don't recommend Superthrive - I used it for years and it does nothing that kelp/seaweed extract (with kelp in it) won't do, i.e they both have vitamins, minerals and growth hormones (except kelp/seaweed is organic and better for you, your plants and your soil).
Also, I (personally) don't have a high opinion of root stimulants, they're there because people will buy them thinking they'll rapidly improve their plants, but all it does is make some nob in an office rich. If MJ can grow 12 foot and strong enough for kids to climb by itself, why does it need a root stimulant? lol ... Feel free to use it, I recommend getting two identical clones, using it on one, bt not the other, and then compare their growth etc. I'd be very surprised if you saw a difference in the final product ... However, if you do, let me know!