Its a yes they can move freely in a unit. As long as they stay in the front rank, and they don't exceed there maximum movement allowance. So as a example if you just march in a straight line forward, and both the sorcerer and the unit he is in have the same movement, lets say they can march 8 inches, then the unit will not march a full 8 inch straight forward, because the sorcerer will spend some of his 8 inch allowance moving across from one side to the other. Usually in some diagonal line, which will mean the unit will move forward as a whole somewhere around 6-7 inches.