I'm not going to get too deeply into the feral cats. Sometimes they are a problem, sometimes not. They can kill chicks but don't always.
How have those 8-week-olds been housed? Have they been exposed to your adults or are they total strangers? My brooder is in the coop so I start straight out of the incubator or from the post office. The chicks essentially grow up with the flock though protected from them while they are small. At 5 weeks I let them out to roam with the flock. My broody hens raise their chicks with the flock. I've had broody hens wean their chicks as young as 3 weeks old to roam by themselves with the flock. Those broody hens spent three weeks teaching the rest of the flock to leave her babies alone. If they had not grown up with the flock I would not do it that way or necessarily expect good results. So what kind of exposure have those chicks had to the flock?
I have a lot of room. An 8x12 coop, two 4x8 shelters, a 12x32 run and over 2,000 square feet inside electric netting. It's not free range but not bad. How big is your run and coop?
It is easier for me to give specific suggestions if I know what you are working with. You might say something that alters my generic responses.
A standard way to integrate chickens that are strangers is to house them side by side for a week or more before letting them loose to roam with each other. Let them get used to the others.
Typically my younger chicks quickly learn that if they invade the personal space of an older they are likely to get pecked or worse so it usually doesn't take them long to learn to avoid the older. So I usually have sub-flocks, the younger avoiding the older until they mature enough to force their way into the pecking order. With my pullets that's usually around the time the pullets start laying.
With so much age difference between your younger ones all this might apply to them or they may actually intermingle quite well. It's also possible your older chicks will mingle pretty well with the adults. You never know how that will actually work out. So much depends on the individual personalities of the chickens involved. What I usually see is that the different groups avoid each other during the day and do not sleep together at night. So they need to be able to sleep in a predator safe area separately from the others.
My hope in this type of integration is not that they are all immediately one big happy flock, eating and sleeping together. My hope and expectation is that they do not injure or kill each other until they mature. Then they become one flock.