Mixing a flock... good or bad?

First, I suggest reading Buff's article. It might help.

Buff Hooligan’s Adding to your flock
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock

How you add the 5 week old ones is the problem and depends on your set-up. Integration is dangerous to the younger ones. The older ones will keep them away from the food and water and will often attack them and seriously injure or kill them. The normal recommendationis to wait until they are about 16 weeks old so they are closer in size to the adults and can better defend themselves. I have sucessfully integrated chicks about 12-13 weeks old with adults, even with an adult rooster and several young roosters, but it took work and time. I raised the chicks in a brooder in the coop with the adults until the chicks werea bout 5 weeks old. Then I moved the adults to a tractor and let the younger chicks have the coop and run. The tractor was always within site of the run. When they were about 8 weeks old, I started letting the chicks free range one day and the adults free range the next. At 10 weeks old, I let them free range together. At 12 weeks old, I started letting them roost in the coop together, but made sure to open the coop and run early every morning before they came down from the roosts. At 13 weeks, I started waiting later in the morning to open up and then only opened the coop so they had to share the coop and run. Part of this was to retrain the hens to lay in the coop instead of the tractor, but part was to gain confidence that I can leave them safely locked in the coop and run together. At 13 weeks, they are still not a truly integrated flock. The adults spend the day outside in the run and the chicks stay in the coop, but I can leave them locked in all day and they don't try to kill each other. It is important to give them separate places to eat and especially drink. The adults will keep them from the food and water if they can. It is also important to have lots of room so the younger ones can get away from the older ones.

From your criteria, I don't think it matters what breeds or mixes you get. Some are better at egg laying than others, but that does not sound like it is at the very top of your criteria. You might look through the Henderson Chart to help you select a few to investigate, then check out some pictures of them on the Feathersite site. The My Pet Chicken breed selection chart can be a good starting point to help narrow it down. For the record, I am real pleased with my Australorp and Delawares, but they are hatchery chicks and other hatcheries will give different results for different breeds. But I'll mention those for your consideration.

My Pet Chickens
http://www.mypetchicken.com/breedQuestions.aspx

The Henderson chart
http://www.ithaca.edu/staff/jhenderson/chooks/chooks.html

Feathersite
http://www.feathersite.com/Poultry/BRKPoultryPage.html

Welcome to the addiction. Good luck!