I have tried the nutes a few different ways on seedlings, etc... And from my personal experience, as soon as you have a real leaf, 1/4 inch long or so, (not those first 2 from the inside of the seed), the plants seem to like the nutes. I do start them at 1/4 tsp per gallon (very, very weak solution) of only the green for 1 week or so, then bump them to 1/2 tsp the 2nd week, then 1/1/1 tsp of each green, red, brn from then on until I start flower basically...

I currently have software issues with my camera... You can see the basic cab I am using, light fixture, etc... in my other threads. It has been painted white inside and now has 5 buckets under MH400 veg and 430 HPS flower.
For now, I just have the exhaust fan pulling air through the glass globe around the bulb. It is vented out the top... Depending on temp issues, another exhaust may be added this grow at the top.

Also, as soon as the plants get a little higher, I may put a small fan on them. For now, they are just at or above the hydroton rocks. I am trying to find a small fan to oscillate on the bulb surround, then on the plants, then up, then down... Instead of side to side. I just am looking at different possibilities.