I didn't post a reply to your inquiry above, but I do take almost the exact approach to chicken handling as the above poster - "abhaya" I have only ever lost two hens to some type of unknown disease and those were not recent. It had no effect on any of the other hens, none seemed to care. I think I was the only one that did morn their passing. However, I presently own around 15 birds counting two roosters, "mutts", and they are from at least three different sources and as many different ages, so mixing them shouldn't be a problem.
Personally I kind of think you are making too much of details, raising chickens is not rocket science. There is far more that I don't know about chickens than what I do know, and I have done fine keeping chickens for the purpose of eggs and meat for many years. While some think of their flock as pets, ours are really just farm animals to us. Not that I'm against having them as pets, I love it when they gather around me wanting handouts, but they have never been what a cat or dog would be in the home. However, saying that, I worked in a home years ago where a woman lived alone and had a rooster for a pet, she kept in the house, in town. When I entered the house, she had to move the bird to keep him from attacking me to protect her. Kind of funny at the time, but that was her pet.
So, relax a bit and just dive into this, sure you may make some mistakes, but most of the time it won't make a difference to your birds. As for keeping young chicks the right temp, I have a box I keep mine in until I can move them to a larger coop, I have 1-2 heat lamps I can use to adjust the temp with. If the chicks crowd around one light and look cold, I turn the other lamp on, if they spread out, they are too hot and I'll move the light back some. Yeah, you'll do just fine mothering your chicks. I only had one chick die and it died because it got something caught in it's throat not from something I did right or wrong and not from a disease. Sometime things just happen.
Good luck!