If those first fish have ICH (not the mollies, Highly unlikely they had Marine ICH) then the tank will have ICH and every fish after will have ICH until the tank has gone fallow (totally fishless,including the mollies now, 72 days)and the fish treated in a QT with one of the 3 effective ways of eradicating it.
But wouldn't I be able to tell if the first fish are infected within 72 days? If I add the first batch of fish (not counting the mollies in there now) THEN wait at lest 72 days before adding any OTHER fish, isn't this the same as quarantining? Essentially, I'm just quarantining in the display tank, right?