If you can safely provide a warm spot in the coop and you don’t have adults out there, there is no reason they can’t go out there straight from the incubator or post office. There is no real difference whether that is in your house, your garage, or in the coop. Many of us do that. The basic idea is the same for a brooder and often easier to accomplish in a coop. To me the perfect brooder provides one warm spot and cooler areas so they can self-regulate. I’ve put chicks in my 3’ x 6’ brooder in the coop with outside temperatures in the teens Fahrenheit. I might see ice on the far end but the end they are on is toasty. If the outside temperature warms up into the 70’s they just go to the far end away from the heat where it is cooler. I use heat lamps but if you set them up properly heating pads, heat plates, hovers, and other methods work great. If you don’t set them up properly they don’t always work great.
When you can remove the heat depends on a few things. What are your nighttime lows? Normal or average temps are not the issue, what are the extremes. That’s where you can get in trouble.
How well is the coop protected from breezes hitting them when they are on the coop floor? You want them to be able to get out of the wind.
How well is it ventilated? You need for good air to come in and bad air to go out. There are different ways to accomplish this but a simple way is to have ventilation up high so air can be exchanged but breezes pass over their heads.
How have you been feeding them? If you feed them a regular chick starter it contains enough protein for them to feather out quickly. If you feed them a lower protein feed or give them a lot of low protein treats they might not feather out quite so quickly.
How well are they acclimated to cooler temperatures? If I’m used to a tropical climate and hop on a plane to a cold climate I’ll freeze, even if I’m dressed correctly. If I’m used to cooler temps it’s not bad. Even short exposures to cooler temperatures helps a lot.
For what it’s worth I raise mine in the brooder in the coop, I feed them a Starter so they get a good start to life, I have great breeze protection and great ventilation up high, and my brooder is big enough they can play in pretty cold areas. I’ve had chicks just over 5 weeks go through nights in the mid 20’s F with no supplemental heat. They can handle lower temps a lot better than many people think, but it can be a little different depending on the conditions they are raised in.
I’m not sure what your current conditions are or what your coop looks like, but if you can safely provide a warm spot out there they should be able to go out today. If you use heat lamps (which I do) throw that clamp away so you are not even tempted to use it and use wire to support the lamp so it cannot fall. Do not use string or plastic that might burn or melt, use wire. I’d leave them locked in the coop itself for a couple of weeks, then open the pop door and let them go into the run whenever they wish during the day. Sometimes when I open the pop door they are all on the ground within 15 minutes. Sometimes it takes 3 days for them to build up their nerve and check out the run. Some people like to micromanage these things but I prefer to just let them go at their own pace. They eventually work it out.