Tenacity is the brand name for mesotrione. The scott product is good, but it the long run, it is way more expensive. It is granular so that makes it easier to use. Once you get to practice and use liquid applications, it opens a lot of different products to use and a lower cost (pennies vs $20 in some cases). If you have the sprayer, just practice with water and some dye until you dont ended with water in the tank (walked too fast), run out of water (walked too slow) or have areas without dye.
For a reno (no grass in the ground), the recommendation is tenacity at seed down and then at 30 days. It really helps from weeds growing in the bare soil.
For an overseed, I dont see the need for tenacity at all. It wont hurt to use it at seed down or 30 days later.
It is still possible to screw up with the granular, but not so much. You need to follow the label. A bag might cover 5ksqft, but you only have 2ksqft. Therefore half a bag should be too much. Ideally you want to weight the amount you need (lb/ksqft as listed in the label). Then you want to apply that qty evenly thru the 2ksqft. If it is not even, then you will end up with double rate in some areas. Dont trust the spreader settings in the bag since it is too general and easy to screw up, weight it with a bathroom scale and a bucket.
Scott starter with meso is designed to be used for new seeds. Follow their label recommendations and you will be fine.