I hatch year round. For me, there is no 'best time'. Since I keep a heat lamp on in the hen house all winter I feel I may as well toss some chicks under it. I should point out that my hen house has a separate area for chicks, safe from the chickens, but the heat lamp in the chick pen heats the whole hen house.

Mixing different aged chicks in one pen is a recipe for a gang fight, with the little guys losing out. Chickens are not great humanitarians and often do not play well with others. The older chicks will very happily trample and peck the new guys, and the new guys, avoiding the bullies, lose out on the food. It can be done, if you have to, if you have no choice. But is high risk and high stress, unless you have a LARGE area with several feed and water stations, way more than you need, so the picked-on guys have a place to go... but even then they want to be with other chickens, even if they are mean.

Lack of chick growing space is the one thing that prevents me from overpopulating myself! It annoys me as I'd like to be stuffing eggs in the bator all the time. But in truth, I just don't have the space, so reality keeps things from getting to crowded!