There is no problem using superthrive for sprouting and cloning. In fact, you should use it. It aids in eliminating stress from first starting out or transplanting. It has vitamain B1 and some other things proven to help in this area. However I would not use as much as the directions call for, at least in the beginning. I use 2 or 3 drops per gallon of PH adjusted water. I am sure some will disagree with me, but I am not only going by what the consenses seems to be, but by what my own experiences have taught me.

Do not worry a bit if the lowest leaves turn yellow and die. This is completely normal when a plant is just starting out. It is actually feeding off itself, using the least important parts. I grow hydro, but it seems like at least half of my clones do this for the first couple of weeks, but turn out just fine.

As long as new growth keeps coming out, and it is healthy-looking and stays that way, you don't have a problem.