It is impossible to combine adults and chicks, also difficult to combine chicks more than a week to ten days apart in age. You end up building separate quarters and can't combine any till they are all the same size - around 5 months old. It can be sometimes impossible to combine adults from different sources - depends on the birds.
I think getting chicks and raising them is a wonderful way to get into chickens. Simply get all the chicks you want at once so they are same age and go in the same brooder box. Their growing up together, they hang out as one flock.
I also wondered how many. My experience - I got a box of 25 because that was the minimum. Another time, I got 2 of every kind at the feedstore because I wanted to see what they were like myself - it came to 25!!!
Both times, 25 or 30 were overwhelming. Each time I gave away hens till I had ten - that was manageable.
A third time, I picked up seven at the feed store. That was the easiest number. It was a breeze, not much work at all compared to my other years. They ranged around the yard and weren't any trouble in that number.
One what kinds for children - brown egg layers and blue/green egg layers are the calmest. All the common ones are good egg layers, just pick the ones you like the looks of.
The different white egg layers I got would explode into feathers beating themselves senseless against the walls and fences when I got too close - they were Ancona, Ladenvelder, and Andalusian. I never could touch them after they were 3 months old. They were pretty, but it was distressing to watch them do that.
Easter Eggers (sold as Ameraucana's by hatcheries and Auracana by feedstore) became my favorite.
Good luck