I’m assuming your RIR’s are from a hatchery? The ISA’s should lay more and bigger eggs, but hatchery RIR hens will should also lay a lot of eggs, just maybe not quite as big. Like Fred said, if you cross the ISA’s with the RIR they will lay somewhere in between their parents. If that RIR rooster came from the same flock as the RIR hens, he should pass on the same type of egg laying genetics as the RIR hens you are seeing. So you should get hens that lay somewhere in between the ISA’s and RIR’s.
The ISA’s are bred to be smaller than the RIR’s. There should not be as much meat on them as there is on the RIR’s. That’s so they can be more efficient in converting feed to eggs. They don’t have to use as much feed to maintain that bigger body. They’ll pass that trait on to their offspring too.
The ISA’s are a four-way cross. Their grandparents each come from a specific flock, so the ISA’s are a jumble of genetics designed to peak in those birds. You are never sure which genetics are going to be passed down to the offspring. I’ll stand behind what I said above but those are in general. There can be some variations in exactly which and how much the different traits are emphasized in the offspring, but all the offspring should show those traits. It’s not just color or pattern here you may get some differences.
If you are happy with the egg laying size and frequency of your RIR’s, breed those so you get more meat. If you want to increase egg size or frequency and will sacrifice a bit on meat, breed your ISA’s to the RIR rooster. Or breed some of both and decide when you see the offspring. You can always weed them out later if you are not happy with the results, plus you can eat the mistakes.
If you want meat, eat the ones you don’t want to eat and breed the ones you do, especially your roosters. For egg laying, breed the hens that lay the way you want them to. Both roosters and hens will contribute genetically to egg laying and size, but roosters don’t lay eggs so you can’t evaluate that except by seeing what his mother and grandmothers laid. By keeping the roosters you want that hatch out of the eggs you want and evaluating the hens egg laying, in a very few generations you should a flock that ties into your goals pretty well.
You can eat any chicken any size or age but you have to vary your cooking methods based on age. Those older hens will need to cook slower and with more moisture than a young fryer. There are lots of threads in this section about how to cook an older bird. Crock pot, stews, roasting in a covered pan to keep moisture in, chicken and dumplings, coq au vin. Lots of choices.
https://www.backyardchickens.com/f/25/egg-chicken-other-favorite-recipes
None of the hens will have a lot of meat on them but the ISA’s should really have not much. When I butcher my chickens, I cut them into serving pieces, thighs, drumsticks, breasts, and wishbone. These are cooked for the table. But I also save the backs, necks, wings, gizzard, and heart for broth. I save the feet too. I know there is a YUK! factor for people with the feet, but with practice if you scald them you can twist the toenails off and peel the skin off. That cleans them up enough for me.
I put the backs, neck, wings, gizzard, heart, and feet in a crock pot along with bay leaf, peppercorns, onion, carrot, celery, and some herbs like basil. Oregano, thyme, and/or parsley, fill it to the top with water, then cook that on low overnight, say 12 hours. Especially with an older bird you’ll get a broth a lot better than anything you can buy at the store. Old birds make the best broth.
I go even further. I pick the meat off the neck, wings, and back, plus whatever is floating around because a lot has cooked off the bone. You have some really well-cooked meat great for tacos, chicken salad, casseroles, anything you want cooked chicken for.
With hens there is not a lot there so when I butcher I might process 4 hens and make three packets of parts for the broth, sticking them in the freezer until I make broth. If you feel there is not enough meat on a n ISA hen to make it worth cooking the parts, use the whole carcass for broth and pick the meat, just be careful of small bones.