Hey there,
I start selling them right out of the hatcher on day 2. You can certainly advertise them anytime you'd like. The beauty of this breed is the auto-sexing ability. Everyone knows exactly what they are getting, whether male or female.
If I was going to keep Legbars for breeding, I would let them grow out until almost laying time to fully examine for breeding quality...that's if you are going to focus on SOP (standard of perfection). The cockerel especially. You cannot really tell until the cockerel is almost fully grown if he will have a short back, tail too high, coloring is off, etc. Just things to keep in mind. If you are interested in SOP, google for characterics of legbars to educate yourself more so you know what to look for.
As for the cockerel with the defects, I personally would not breed him. You do not know if that was genetics or not and don't want that passed on. I stopped helping chicks hatch a long time ago. Every single one I assisted didn't live past a year. I feel like it's mother nature's way of telling us, 'something is wrong' and she deals with it. Of course, this is only my opinion and experience. You have to do what is right for you