I think that if you read the label, they recommend that you do not use it until you have mowed the new grass three times. Since you say the new grass is only 1/2" to 1" high, I hope that means that you haven't mowed it at all yet.

I suspect that by the time the new grass grows enough to have been mowed three times it will be long past time for a pre emergent to do any good.

The grass seed is already up so you can use Halts when the first trees or shrubs break into bloom. Forsythia is usually used as the trigger for preemergents.

The reason for waiting to fertilize (as mentioned above) is so you are fertilizing actual grass and not just the weeds.

Wait until your grass gets up to 4 inches and then mow it down to 3 inches. Keep your mower there and don't lower it unless and until you decide to overseed. Then you can scalp it.