i put mine in when they were on their second set but i still had to hand water the rockwool until i seen roots popping out the bottom of the net pot because you dont want the water touching your rockwool.. it needs to be able to dry some of you will have bud rot issues if a majority of the roots is still in the rockwool, lots of people put in to early, the stronger your root system the better chance it can withstand any issues when needing to transplant in.. none the less if done properly and depending on your DWC setup you can do it now or wait till your roots or a lil further out..
and regarding nutes i would go with quarter strength starting off and increase as ur plants tell you , defenitely need a PPM / EC meter. . until you can talk to your plants and listen to them ..
pics would also speak a million words.