I've got a mixed flock too. I wanted heritage breeds with good reputations for disease resistance, foraging skills, and predator awareness, to suit the open conditions here. I chose different breeds to get different coloured eggs, and it helped me know who was laying what, but only for the 1st generation. It's very difficult to tell apart the eggs of the home grown hybrids, as they're mostly various shades of off white to light brown
Besides the home grown chicks, I've acquired either pullets (first 3 years) or hatching eggs (more recently) in groups of at least 2 and at most 7, depending on broody and available coop space at the time. But predation and other issues have reduced the numbers of each type to between 1 and 5 of a pure breed kind. In either case the clutch grew up together within the flock, and separated themselves as juveniles, then mixed on reaching maturity. That's true of the home grown hybrids too of course, who all look different from one another and their parents. There is room for subflocks here, and they do form, but they're always temporary. Roosting is the real giveaway: there's no consistency in who roosts with whom or where on any given night. I know because I look into each coop each night when I close up. I think it might change if the flock grows bigger than 20 ish; that seems to be a threshold for a flock before it splits.
As I grew more knowledgeable about chickens and their offspring, I thought more about what genes to add to my flock's mix, explicitly with a view to increasing its diversity. My flock now consists of breeds that developed, at least originally, in England, France, Netherlands, Spain, Sweden, and South America. I have endeavored to source them from breeders with prize winning stock when I can, and I'm happy to take stock they don't want to grow on as breeding stock. Of course with very rare breeds beggars can't be choosers, and I've been disappointed with some; I think there's a good reason why some of these rare breeds are rare. Others may simply have been inbred too long. Those constitutionally fit for this environment thrive, and reproduce. The offspring are different sizes and shapes and colours - one even has one green toe on an otherwise yellow leg! - which is fine, because I'm not trying to develop a breed, just a flock that does well here. It won't be RJF, but hopefully it will be something suited to temperate woodland, with a broad range of genetic resources to face whatever challenges are thrown up here.