if you are successful at getting them to root you will see the roots coming out the sides and/or bottom of the cubes in 5-15 days or so... i usually plant 'em into soil as soon that happens...
Only if you don't mind giving them N-BURN.... Like WhiteBerry said, not for a couple weeks. Better yet, let the plants speak for themselves, and tell you when its time to eat... when the bottom leaves start to yellow a bit.. THEN its time for some food.
Special supplements in the mist water.... now thats another story. And entirely up to you. Superthrive, Seaweed/kelp extract, humic / fulvic acid, GA3, Brassilanoid, worm shit, etc. I am currently misting some clones with a very mild addition of Fulvic, seaweed extract, GA3, and Brassilanoid {sp?} I went mild enough that I dont notice a huge difference... not enough to jump 3" of new growth overnight... but enough to be slightly interesting to the clones.