Some of them would almost certainly be mixed. Breeds are a man-made thing, the chickens don't know or care. If you want pure leghorn eggs you need to separate the hens from the Cochin. Now, how it works.
It takes an egg about 25 hours to go through the hens internal egg-making factory. It can only be fertilized during the first few minutes of that journey. That means if a successful mating takes place on a Tuesday, Tuesday's egg is not fertile. Wednesday's egg might or might not be, don't count on it. Thursday's egg will be fertile.
A rooster does not mate every hen in the flock every day, but he doesn't have to. After the rooster is finished and hops off, the hen stands up, fluffs up, and shakes. This fluffy shake gets the sperm into a container near where the egg starts its internal journey so it can fertilize the egg. That sperm can stay viable in that container at body temperature for 9 days or as long as over three weeks. Most of us count on two weeks.
This means if you want pure eggs you need to keep the hens isolated from the roosters you don't want for at least thee weeks. A lot of people use three weeks and it usually works really well. Some people go four weeks to be absolutely certain.
My suggestion if you want pure chicks is to break that Cochin from being broody, there is a good chance he will go broody again later. And isolate the hens you want eggs from from the roosters you don't so you have eggs ready for the next time. That isolation may involve more pens or removing the Cochin roosters from your flock.
