Where do you live?

Most of Zone 6 is a northern area for cool season grasses, so I'll focus on that. Kentucky bluegrass is a sod forming grass requiring full sun. Once established it will quickly fill weak, dead, or damaged spots. Fescue, the other popular cool season grass, is a bunch type grass that forms individual plants. It spreads by enlarging the plant itself as well as tillering. Tillering is a very slow process compared to the speed with which KBG can spread.

Have your temps dropped from the summer highs, yet?

If you apply starter fert now, it will be gone before you have any new grass roots to take it up. Milorganite or other organic fertilizer is always a good idea.

I am in Central NJ. I'm guessing I should do a mix of TTTF and KBG. Not sure of any special seed sellers around here. I'll probably try Jonathan Green over Scotts, right? Temps look to be going down to the high 70's to low 80's for the next week-10 days

Yes, you are easily good to get going on this project.

With seed you get what you pay for. The very best is only available online or from specialty seed distributors. You can recognize it by reading the Guaranteed Analysis on the label. Look for 0.00% weed seed and 0.00% other crop. These are percentages by weight not by seed count. The real problem seeds are the weight of dust particles, so you can easily have thousands of them in 10 pounds of seed. On the other hand, most weeds in cheaper grass seed can be dealt with using conventional sprays later next spring.