I have started seeds in Jiffy pucks, and in solo cups of Fox Farms Ocean Forest soil, and or Roots organic soil. I have started seeds in coco, but it is easy to lose the seed...had quite a few sink farther into the coco than they could grow up before they started to die, so I don't recommend a solo cup of coco, but a small cup of soil does great...or the JIffy pucks.
I haven't used Rockwool personally...not even for hydro...It just seems too picky for my style of laziness...