you can use ferts when the plant has 4 sets of true leaves, if you fert before you have a chance at burning.

when the plant is young it has enough food in the plant to make it grow

if im not mistaken the big grow is only a 2 part feeding system and i hear it is better to use a 3 part(with micronutes)as you can control your nutes better

some people feed feed water, and others feed and water, but its up to the plant on how much feed it can handle