In 15 yr or so I have never had my rockwool melt and I saw a few extreme alk/acid flux and never saw it melt so I think dude is just speculating the cause of degredation which it does on it's own buy the end of your cycle. I sure wouldnt do a re-veg with it though. Now that is for a hydro setup. If you are refering to a start cube to soilless then dont worry bout it soo much. mixing medium is not at all recomended but that particular combo wont at all be detremental, maybe a little troublesome. Try a peat puck next time. I concur with 5.8-6.3, 5.8 being optimal for hydro IME.