I've used plastic cups before with no problems, I usually use Styrofoam cups then tape around them with black electrical tape just to make sure the roots don't get any light. A little extra security I guess.
But like I was saying before have always used miracle grow soil mainly cuz u really don't have to feed them that much through out the grow, just a little cuz the nutes in it take care of most of it. I don't know what the deal is though probably just a few bunk seeds I guess. :S3: