be very careful that your plants are getting adequate air flow. Seedlings are very vulnerable to a rot called 'damping-off' or pythium wilt, which causes them to get thin rotted stems and fall over, and comes from too much humidity. You should start opening the dome for about a half hour at a time, a few times a day, to get them used to normal air and prevent this. I never put seedlings under a dome personally.
They are ready to feed when they have 4 sets of adult leaves and are working on the 5th.