So does that mean the littles are already in contact with the bigs? I'm confused about that part-- if so, sounds like the bigs are just ignoring them. In any case, our setup is that we have a chicken coop, and then we have a fenced area outside the coop that is just cattle panel so only 50" tall and wide openings, and there's an opening in that fence that lets the chickens out into the rest of our property for free-ranging. I was very freaked out about my littles going out the pop-door of the coop and into the fenced part, so I put chicken wire around the whole thing to keep them inside it. So, two things:
1) They didn't just immediately tear out the pop door. Once I let them roam outside the brooder pen, it took them at least a week to decide to venture out the pop door. With your setup though, it could be earlier, not sure. But once mine did go in and out, they had no trouble at all remembering/knowing how to get back inside.
2) Once they went outside the pop door, they were very careful about where they ventured for about a week, and stayed really close to the coop and went in and out a lot. Then they were ready to start exploring, and they discovered they could fly up above the chicken wire and jump back down the other side. So after the first week or so, the only chickens I was keeping in the yard were the older ones. So I opened that outer door and everyone started free ranging, again, the littles being very careful how they explored but nevertheless doing it.
So, yes, the chicken wire kept them in for another week or a bit more, but it would need to be 36 or 48 inches high to go longer than that. I made the mistake of only using 24".
You can use a nail to twist the chicken wire quickly onto your fencing if you want to do that. If you will have groups of chicks in the future it will be in their interest, but it should be 1" diameter. I figure, they grow so fast, that each week of protection they got was enabling them to survive a bit longer. So to sum up, my chicks went out the pop-door to the yard at about 3 weeks, and over the fence to the free-range at about 4 maybe 4.5 weeks. I lost one to a feral cat we are no longer being troubled by, everyone else grew up.
I reported on the exact ages and so forth here on this thread, but it's been long enough I don't remember exactly.
hope this helps,
--Victoria
Well, they are in contact with hardware cloth in between

The fencing I have is the galvanized wire fence with 2x4 inch openings, so I will most likely have to put chicken wire on. This is my first time with the MHP method but I've raised and integrated chicks before. I'm just not sure of the timing although it seems from what I'm reading that they will be able to be in together sooner than when the chicks are in a brooder completely separate from the older birds. In the past I have used wire dog crates and pen, both of which have smaller openings in the wire, but it would be nice not to mess around with all that.