Ok, phew! Murray McMurray got back to me after I sent a picture and assured me it's not one of their Cornish X birds that take special treatment!
Email said it was most likely either a Red Ranger or a Pioneer (Dixie Rainbow), though the also do sell RIR. The first two are supposedly multi-purpose--Roos can be slaughtered at 12 weeks for "market value" and hens lay large brown eggs.
Phew! I know that I'm going to have to part with all but one or two of the Roos, but I just didn't want to be feeding and caring for the birds improperly.
Thanks for all of the responses!
And no, I don't have any particularly large yellow or white birds. These buff with brown variegated feathers are the biggest I have. I do have smaller solid light yellow chicks--maybe White Leghorn? No bigger than the others. The 27 mystery chicks were a week old yesterday and my breeds that I specifically ordered are a week today, so in theory, they're all essentially the same age.