I've always kept mine apart until PoL-mainly because they are eating different things of course but also because of what the other posters mentioned. This year, for various reasons I had a broody hen hatch in an arc in the bigger pen, where they were kept until 7 weeks old. Hen was then fed up and I let them all in together-there's only one pullet, the others are all boys destined to be culled. The two cock birds and all the other, older hens have been absolutely fine-one male saw off the top hen when she had a bit of a go on the first day and thats it. Broody hen moved back into the coop on the first night and the growers kip in the broody arc and although they followed her about a bit for a few days, that was it. However, the older birds had seen the chicks from the get go.
Cats, dogs (and sparrow hawks!) are different though-not sure on that, I wouldn't expect a hen to defend successfully or even that aggressively once the chicks are a few weeks old. I wouldn't expect a grower to be able to chase a predator off or a dog/cat to not be excited by a young grower. I would see it as your job to keep the dogs/cats away.