You don't want to use a weed killer when it's hot because the weed killer will not work as well on the weeds, but it is also more volatile and may damage plants that you want to keep (such as flowers, shrubs, trees). Typically, you should wait until you've mowed at least three times before using a weed killer on grass, so you should be well past that by the time it cools off enough to use a weed killer.

What kind of grass do you have? If it's St Augustine, check the label carefully to make sure you use a product that won't kill the grass.

I'm not a fan of the combo fertilizer/weed killer products. I'd rather use a fertilizer first, get the weeds growing vigorously, then hit them with the weed killer so it's more effective. I'd also rather spot spray the weeds than blanket the lawn (but if you've got as many as you describe, spot spraying may not be the best option this time).

Silly me, I failed to mention that yes, I have St. Augustine.

Spot spraying would take forever, although there might be a few places I could do that to. I will definitely wait until we're back in 70's, or would 60's be better?

So if I fertilize first, how long should I wait to put the weed killer down? Maybe a week after?

I think a week after is fine. Just make sure to check that the weed killer is okay to use on SA. I'm pretty sure it's sensitive to many of the weed killers (I'm not a warm season grass guy, but I've read of people using the wrong thing and killing their lawns).

As per instructions at a nearby plant place-I put Down preemergent, and then about 3 weeks ago a dollar weed chemical via spreader. It was suitable for use with St. Augustine. No results, so have scheduled removal, tilling, top soil and New sod to be laid down in a few days. Will my prior treatment kill my new grass? if I need to cancel the order, how long do I wait for new Sod?