To breed an ISA brown you would need to have the parent stock breeds used to make an ISA. They are not a true breed but a hybrid (as you point out). Breeding an IS rooster and hen would likely produce offspring that were not true to the ISA characteristics. I do not believe you would have any particular disease problems wit an ISA under good management. I have not been able to determine where hatcheries get there stock but I would wager that it is from hatching egg producers.

Anything you cross with your ISA browns will no longer be the high-producing hybrid. But if you want to experiment - go ahead.

Be careful bringing in any adult birds though - you don't want to bring in disease with them.

Quarantine any new birds for three weeks (and care for them last), then introduce a few hens with the new rooster and wait another three weeks before combining all the birds. If the original hens get sick when introduced to the rooster, the rooster is carrying a disease your hens are not protected from. If the rooster gets sick, your hens are carrying a disease the rooster is not protected from.

I get many calls from people who purchase a new rooster and the rooster is fine but all the hens died. So please, be careful when introducing new birds to an existing flock.

Introducing outside birds that have potentially been exposed to disease (i.e., not chicks) is a potential problem. Always quarantine any new birds.