Good breeders in what way - simply producing offspring, being good to their hens, etc? As mixed breeds themselves, your roos will be producing further mixed breed chicks, so as to "good" in the type of chicks they will produce there is really no way to tell as it's a mixed bag what you'd be getting....especially since your ISAs are also mixed breed birds.
As to the white bird - a picture of the bird standing up where we can see the whole bird would be helpful....as would an approximate age, if you know.
