Get the Scotts starter with weed preventer (mesotrione), it's worth the extra bucks. Unless you already have Tenacity on hand.

I have the Scott's starter with mesotrione and plan on using at seed down. When should I apply again? I also have plain Tenacity, should I apply that again at some point?

When seeding, only apply tenacity at seed down. Don't apply it to new seedlings until at least 30 days after germination is complete. (That's not the same thing as 30 from when you first see germination starting). But IMO it's kind of a waste to do that. 30 days after germination completes, you've probably already mowed at least once. At that point, you could just wait a week or two to mow again. At that point, you're ready to use regular pre-emergents and/or herbicides which will provide better weed control.

If you're overseeding into an existing lawn that has thinned out a little I prefer no fertilizer at time of seeding. IMO fertilizing at time of overseed just causes the existing grass to compete with the new seedlings. The seed itself contains everything it will need for the first few weeks after germination. I will generally wait and do a fert app after the second mowing and have had excellent results.

If you are doing a full reno or starting from scratch either way will work but I still tend to save the fert for after the second mowing.

I agree with forsheems. Fertilizer only works on plants that have roots. New seed won't have roots enough for fertilizer for at least a month.