I'm sure you will hear opinions from both side of the fence on any of your questions.
I was new last summer, and never really new about this site to ask questions and did good by following what the hatchery suggested to do. Started off with 15 then bought 3 more and then another 20. All are doing well except one that was killed by a hawk.
Warmth is key in the beginning other than the common water food thing. Water though you want to make it so they won't accidentally drown.
Watch the chicks and if they are all huddled up they could be cold. If they are away from the heat lamp or what ever you may use chances are they are to warm.
As far as mixing the difference in age thing. Many different ideas on that and they all worked for someone but it might not work for everyone. I've asked a couple times because I am going through that right now introducing 3 into 14. I was told keep an eye on them and as long as they don't get to beat up to bad by the pecking order, let it play out.
Really don't know what to say about weather you should or should not do this practice thing. It never even entered my mind when I decided to get into raising chickens. Sorry.
I went for egg layers and have 6 different breeds. All did well, and all in the coop now. Separated with a fence between the first 17 and the newer 20.
So much on this site to read. So many different opinions to choose from. Pick one or combine a couple and try it out. But read, read, read!