A couple questions first has anyone had any luck growing seedlings that were stuck in the shell the 3-on the right are all the same age and 2nd the one in the middle looks unhappy fox farm soil haven’t done a soil ph or ppm haven’t given them enough water to get runoff yet
Everything looks good to me too. Different phenos will grow at different rates so I wouldn't get too caught up with growth rate. If they are all different strains then even more reason for them to grow at different rates.
The one you mention with the browning on the cotyledon leaves may just need some air in the root zone. Let the medium dry a bit and you should notice an increase in growth on that one.
It happens, she should be ok. By time those two leaves die off you should be a couple node up and a few fan leaves.