I try to wait until I have 2 sets of true leaves going then I give them somewhere between 1/4 and 1/2 strength fertilizer. I have been using Jack's Classic 20-20-20 and it seems to be working, I have some good size plants for as young as they are. My Peter peppers are about 2 months old and they are about 7" tall already!!!
I fertilize, then water, then ferilize then water. Just alternate back and forth on feeding/watering and you should be fine. I wait until the top of the soil is dry and the dirt is pulling away from the edges of the solo beer cups I have almost everything growing in.