i think its too late now to do any flushing.. alot of people love to slam miracle grow soil, but its not as bad as some people may lead you to believe.. just go very light on any nute feedings that you may give your plants for a month or two, and you should be ok.. your best bet for miracle grow is to use some dolemite lime, about a cup per bag, to help buffer the ph, which is where most of the problems arise from..
goodluck with the grow..