I'm not sure if I understand what you're saying but anyway:
You're often able to find rare colors or breeds that hatcheries don't carry and/or eggs are often cheaper than buying chicks but there's a bit of a gamble involved since eggs don't ship well. You also need to set at least twice as many eggs as females you want since about half the clutch will be males. You also need a plan for those extra males.
I don't understand your question, but hatchery birds *tend* to be smaller than breed standards but it depends on the hatchery, some hatcheries do breed for the breed standard, you just have to look. Size also depends on early nutrition through the growing months.
You ask them. Most people get their stock from hatcheries or other breeders. And by breeders, I mean someone who is breeding birds to the breeds' SOP from different sources, including good hatcheries, to improve their line; not someone who buys stock wherever they see it and just lets them do their thing without quality control.
A reputable breeder will have worked on their line for some time.
If you want to incubate if you're serious about it, check breeders' clubs or ebay.
For the clubs, look up (for example) Wyandotte breeders club and look for a list of breeders and start looking for someone who has stock you like and is willing to ship eggs. Most people ship eggs since it's cheaper than chicks.