Well, they don't look like Cornish crosses to me, but keep an eye on them. If they start gaining rapidly and looking ugly, then that's what they are.
Columbian is a color variety, not a breed. You said they are getting black and white feathers, so more than likely they are a layer breed. Plymouth Rocks, Wyandottes, Leghorns, and others come in the Columbian variety.
At a week old, they are still young enough to add another chick. If you can find a silkie chick that is a few weeks older, they may integrate better since the silkie will be more comparable in size. The acreage that you have should be sufficient for avoiding bullying, but not a guarantee.
Try looking on craigslist for silkie chicks available.
They look very cute! Hope everything works out with them, and that you are able to find a silkie! They are a wonderful breed, very sweet.