My order from McMurray was 50 blkstars & 25 cochin bantam fancy. I never 'separated' the order, per se, other than having the blk stars in one cage and the cochins in another, side by side.
The cochins got AE or whatever it was, displayed the same symptoms so many other experienced here and started dying one after another. Out of the 25, 5 'survived' and 7 never got sick. I put all the chicks (blkstar included) in a 'juvenile coop' in the hayloft at about 6 wks. The five recovered chicks were pretty crippled in one way or another and soon perished. They just couldn't navigate with the normal chicks. The 6 remaining cochins (and one mystery chick) are doing fine.
By the way, I had one sort of beat up blk star chick that I originally put in with the cochin chicks and it never got sick, fared well, and went back in with the other blkstars at about 6 weeks old. Amazingly, I think I have 5 hens & 2 roosters (the mystery chick is a roo). The group is now 8 weeks old.
Also, FYI, McMurray was a little slow to respond ... but when they did, they replaced my entire order of cochin chicks at no charge. These chicks are now going on 2 wks old and doing great.