slow growing plants and ferts questions....

i got two quick questions. my plants are 3 weeks old and only have the first set of true leaves (most of them). have u guys ever had plants that dont start out grwoing rapidly, is this rate extremely slow for outdoor plants? maybe they are just slow growers, cuz i dont see how i am doing anything wrong (just basic potting mix and some perlite and lil humus). did ur slow starting plants end up alright, or did they die? also, when should i start to fertilize. i have heard contradicting views on this - anywhere from 1st week to 1st month. my plants are 3 weeks old and growing slowly, should they be fert. yet? some of the bottom leaves are turning yellow, i think this means they need ferts...am i right? also, what is ur favorite fert? someone told me basic tomato fertilizer used in 1/2 the concentration, sound good? ne help is appreciated.
PS - some of the plants jus were transplanted from red cups to 8"inch pots, does this mean i should hold off on fert for a while? thanks

 

Plants need food starting on day 12.

Use a full-spectrum nutrient and start out at 1/4 strength. The next feeding ramp it up to 1/3, then 1/2 the next time, then 2/3, then 3/4 and finally full strength when the plants are bigger.

It's normal for the first 2 oval leaves (cotyledons) to die off.