First of all, generally speaking, you can mingle different aged chicks before they're a month old. After four weeks, they begin to take note of differences. But even so, you shouldn't have too much drama mixing chicks within two weeks of each other. So go ahead a put them together. I wouldn't expect they would even notice there are some slighter bigger or smaller. A single peck on the head of a newcomer is normal. After that initial greeting, they should settle in together.
If you have a coop without any adults to contend with, by all means move them in immediately. As long as the days are over 75, they may not even need a heat source. Of course, at night they will until after four weeks or so, and they have feathers up to their heads.
My three-week olds are in the grow-out pen with no additional heat during the day, and a heating pad cave system inside their coop at night turned down to the "low" setting. They're about half-way feathered out, downy heads still and only a strip of feathers on their backs, and they seem very comfortable with no additional heat until dusk.
Those heat guidelines are over-kill in my opinion, chicks generally preferring much cooler temps, in my experience.