Give them some time to establish. 6 inches tall is not a tall cortaderia plant. Was it 6 inches of green foliage, or 6 inches of cut back foliage?

Hard to say anything without seeing them. If they were field clump divisions, cut back and packed in soil, they will need some time to establish before you'll see any growth.

How hot is "too hot" for splitting a 4 gal. plant in zone 8 from pot to (future) fence row?

I'd keep it in the pot until next spring. It's getting close to bloom time, and it's not a good idea to divide a grass while it's blooming. Be sure to either sink the pot in the ground or bring it into an unheated space for winter, and water it through winter.

Another option would be to plant the whole grass now, and then dig and divide it at haircut time next spring.