How do you define "having a hard time"? Pretty sure I worry about the cold more than my chickens. We get to about -25C, and our coop is insulated plus I have a heater I turn on when its double digit cold. Keeping them dry is probably more important than warm tho, so watch the humidity.
I believe all flock is pretty much layer, without the extra calcium. My store doesn't have all flock either. My options are 16% layer, or 16% grower (no extra calcium, but crumbles instead of pellets, which mine waste more of). I get 18% layer for winter.