You wont need to fertilise just yet, the seed has all it needs for the first 2 weeks, but seasol or some type of seawead extract should be applied from the start, it will give a great boost.
I dont know what happened to your cotelydon, but it looks like you have the first set of true leaves coming in.
Is that yellowing on the edge of the fallen leaf, its hard to tell. Is there a chance your pot is holding to much water, it should be moist, not wet, not dry either. How big is that pot, I would start them in smaller pots too, if the roots cant fill out the pot quickly enough the soil can become stagnant, which is very bad, and your leaves will yellow as a first sign.