FYI it went pretty well. We placed bricks (regular little red bricks) on their side to lift the plastic fence 3-4 inches. The little chicks didn't seem to notice, but the hens sure did. They managed to climb under right away. They were more interested in the space than the chicks. Eventually the chicks moseyed over to the big side. There were a few scuffles, but nothing major. The rooster (BR) appears to be something of a jerk. For the most part he ignored them, then randomly would chase one down and try to jump on it feet first. The chicks were fast enough to get away except for once when he got a mouthful of tail feathers. I'm not sure if his intention is to fight, mate or intimidate.
One hen (BR) randomly ran towards them causing them to scurry just to prove she could it seemed. She scared the daylights out of them but didn't harm them.
The thing we weren't expecting was that they pretty much just traded sides of the run. The chicks had a ball jumping on the assortment of roosts and ladders and such on the big chick side and the big chicks couldn't get enough of the little coop/brooder, dust bath (one on each side) and chick food.
We're going to try again tomorrow. We could remove the fence if we weren't concerned about the rooster's intentions. When they run at something then take a semi-flying leap feet first is it always fighting? Is it normal, just give it time or is it time to cull the rooster? I've never seen him do that before and it's my first time being around chickens.
Most of the day he did just fine with them. Maybe the little leghorn called him a bad name or something. He did it maybe 4 time throughout the whole day to different chicks. He just managed to get a mouthful of the leghorn.