I am not 100% sure on this since I am just a homeowner myself but I thought that weed and feed worked better once the weeds are there. Then you apply it when the yard is moist from either dew, rain or watering it so that the application sticks to the weeds and they absorb it. This is just what I believe I have read and understand how it works.
My yard is a dandelion patch right now and I plan on getting the weed and feed down this week since its about 6.5 weeks since I put down the pre-emergent with fertilizer down in March.
Also I believe you will have trouble getting grass to start unless you put soil down since the pre emergent doesn't allow anything to germinate.