I have used Nutrena duck grower/maintenance ration, then when I could not get that, switched to blue seal poultry grower/maintenance feed. I have considered using half layer rations as well, but we seem to be doing very well with the free choice oyster shell, and greens, and whatever bugs they can catch.
Sometimes I turn them out in the garden beds to forage there.
I think chicken scratch is just not a good balanced diet. Perhaps if they needed some extra calories in winter, in addition to a balanced ration, I might think about it, but cat kibble works fine and I think it has nutritional advantages.