You actually need the flowering to overlap so that they different cultivars can cross pollinate each other because they are not self-fertile. Also have to prune all the grafts so that no single one takes up all the sunlight/water/mineral and then also have to prune if you want the tree to be short like I do. peaches and nectarine are the same species just different cultivars, I can see you getting
enough chill hours so no worries on that front. As for late frost, I think that whether you live in Maine/Canada or Florida late frost is always an issue. Hopefully you will get some 1st hand account of cultivars that work for your location.
Iterations are fine, we don't have to be perfect
My 2nd Location:Florida HardinessZone:10 AHS:10 GDD:8500 Rainfall:2in/mth winter, 8in/mth summer, Soil:Sand pH8 Flat