Usually when you grow from seed you want there to be a nice taproot showing and hopefully the first set of True Leaves have appeared too. You don't really want to feed them much at all but if you think they need it then you cant really go wrong with a very low EC/PPM Formulex is a great way to treat seedlings if you can get it.
There is a bit of confusion with Rockwool and it seems alot of people misuse it. Its a very easy medium to over water. In the times I have used it I have found that when going from small cubes to bigger ones the best practice is to make up your nute mix (in your case a very weak one) and soak the larger Cube in the soloution and then take it out and give it a squeeze so the cube is not water logged. You have to think of the way plants work really. They will go searching for moisture and making them work a bit for their feeds is not a bad thing really as it makes them stronger.
Once the big cube is nice and damp just dip the base of the small cube in your nute mix and you will see its like a sponge and it should suck water up. Don't make it too wet otherwise you will slow your plant's growth down if its being strangled by water. Plop it into the larger cube and give the larger cube a pinch on the sides so the small cube is nicely seated. Remember not to break the Taproot so just be mindful not to be too rough. Don't worry if there are gaps.. or anything, the roots will do their own thing. I have added some pics below so you can get a better idea. Just remember not to over water or over fert in rockwool and you will be fine.. Less is more!
This is probably too early to pot up
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This is usually okay to pot up and feed very weak nutes
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In a few days you should see this if you don't over water and can be transplanted into your hydro system.
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