I think as long as they are feathered, you can pretty much treat them like reg chickens. I have 22 chicks that are a week and 1/2 and are starting to feather nicely they will go out next week, if all goes well.
In Portland, I would be more worried about rain than the cold. They can handle cold temps, not wet and cold. Keep them bedded good and keep them dry, you will be fine.