True, good advice. But you must maintain strict cull criteria otherwise what could be a promising dual purpose flock can quickly revert to smaller, longer maturing and less flesh. I try to emulate commercial meatbird producers and have found in breeding manuals hatching chicks out-of-season is possible, but may compromise fertility, hatch rates and chick quality, as well as cause flow-on effects for the flock as a whole. The anecdotal experience of master poultry breeders suggests hatching chicks during the fall and winter months — and even during summer — may not produce optimally healthy chicks that thrive to the same degree as those hatched in spring. Master breeders with no agenda to disparage hatcheries recommend against hatching eggs at any time other than spring. They also advise to rest young hens over winter and spring egg yolks will be much bigger - big yolks produce big chicks which produce well-fleshed meatbirds.
The spring and fall thing is true. Chicks hatched in spring are larger than those hatched in fall, but if you have a rare very expensive breed I would suggest that you hatch even in fall, because the genetics is there. You want to hatch every egg that you can. Once you have made a sustainable flock then you should stop hatching in fall.
Once you have established flock then you can select for other things. For example I only hatched eggs that are laid before 9 am. Because I free range my birds from 9 until sunset. After 7 seasons of applying this selection pressure I stopped finding hidden nests in my orchard.
That being said I still suggest hatching from atleast 1 year old hen and preferably 2 years old hens. Because they are proven birds, proven for breed characteristics and longevity.