Base for flagstone pad; seeding fescue also

I'm wanting to put down a 10X10 flagstone base in my front entry way. I normally use decomposed granite as the base material, but I'm wanting grass to grow in between the pavers instead. I plan on top dressing the whole area around the walk way with compost and seeding some fescue and see how that works out. It's maybe 500 sq ft total and I'm using it mostly as a experiment area. Should I just use cushion sand as the base so the grass can root a little easier or should I still use decomposed granite?

Also, this is my first go with grass seed. I've applied the first dose of glyphosate, and will apply another round later this week. After that, I'll bring in 2 or 3 yards of top soil with some sort of nutrients in it, and seeding then waiting. What other steps can I take to give the grass a fair shot and to reduce weed infiltration?