hey bro, firstly you have some lovely plants there, they are going to grow into monstors, I hope they are spaced away from eachother. If they aren't I recommend LSTing them in different directions from eachother (for light and oxygen, etc).

I was always taught you shouldnt nute a plant before it's got its first set of "adult" leaves but if the soil your working in is low in nutrients, I'd want to find a good vegging nute and try it in very low doses to start (you can't hurt adults with a low dose, but be careful seedlings are sensitive as I'm sure you know)

Peace 2 u my friend,
Denial