You may want to keep records on how big they are at each age. Any chicks will tend to grow faster at first, and slow down as they get older, so you might find that you do want to butcher them at a young age (fast growth, tender) rather than continuing to raise them to a bigger size (never as big as typical Cornish Cross, but bigger than if you had butchered the same birds at a younger age.)
You mention mothering: are you wanting to have broody hens hatch the chicks, instead of using an incubator? Dark Cornish are more likely to go broody than White Rocks, although of course there are no guarantees about which specific hens actually will go broody.
And of course all the chicks will be edible, even if they don't grow as big or as fast as you hope.
I assume you are planning for sexlinks? Red rooster x white hen gives red daughters and white sons, assuming the white mother has the "right" kind of white genes. You should know for sure after the first round of chicks hatch. If all chicks are white, including both males and females, then you have the "wrong" kind of white hens to make color-sexable chicks (sexlinks), but the daughters should still be good layers and the sons will be just as edible as ever.
(If the White Rocks have the right genes for white, you will also get color-sexable chicks when you cross them to a Dark Cornish rooster. Again, red daughters and white sons.)
It's a fun way to use that time
If you want color-sexable chicks in more colors, you can run a mixed flock of white hens and barred hens (like Barred Rocks) with the Rhode Island Red rooster, and chicks will hatch in four colors: red pullets and black pullets, white cockerels and cockerels that are black with yellow dots on their head (white barring will be visible on the black feathers as they grow.)
I did that one year, and it was fun to get 4 colors of chicks from the one pen, all sexable at hatch.
Years ago, I tried exactly the same idea-- but the hatchery sent me Dark Cornish BANTAMS instead of standards. They were cute little birds, but not the right choice for making meat chicks! They are impressively heavy for their (small) size, because they have such a round body shape and their feathers are close to their body instead of fluffing up to look big. They turned out to be nice broodies too.