I wouldn't use a 600w for clones, I think it would be too much for 'em. I don't know how big they can go - I've seen them over six inches though, so anything that size or less you should be good with. Rockwool - I just keep hearing too many horror stories about it. The ph not being right, it stays too wet, I wouldn't use them. Go with either the Jiffy pellets or soil.