what a great observation for a first grow.Day by day.You are definitely right,600 ppm roughly equals to 1.2 EC which is pretty high for a seedling but you've already seen it so there is no problem.Cotyledons getting yellow usually means hunger.Yours don't look hungry as I can see cotyledons in the last pic.Just try to control feeding routine and it will thrive in no time.0.8 - 1 EC is optimal for this stage,then you can increase gradually over time.You can check out my blue dreams on my actual diary.Good luck