I would start feeding lightly once the leaves emerge.

Sounds good, nothing leaf wise just long spindly stems with a seed on the end. Probably an inch tall now.

be careful not to over water them. rockwool holds a lot of moisture.

+1 on not overwatering with RW. i tend to not add nutes early on, let the plant struggle a bit and throw off as many roots as possible. they'll transplant better.

These are dang near 2 inches long now. Very leggy. Maybe 3/4 inch across at the leaves.
Can't wait until we can transplant.

Do these look right for 12 days? Seem awfully leggy. It is lettuce. Think I can put them in a system at this size? I am just paranoid never done this before.

Those look pretty terrible. I think you should throw them out and start over.
You container holding the rockwool should not have any standing water in it.

I have been watering them and pouring off the excess. Had just done that. It is almost like they sprouted and immediately began to bolt. I have never grown this type of lettuce before. I sprouted a bunch of them in a starter bed. If it is dry enough I will look tomorrow and see what the ones started in dirt look like.

How do I keep this from happening again?

To keep them from getting leggy, make sure they have more light. if indoors place a fluorescent bulb directly above them (1"-2")