Oh, good heavens, move them into the coop! Six weeks old, they're ready!
Start acclimatizing them by turning off their heat lamp! Then make the room cooler each night for the next several days. Then move them into the coop. Just be sure there are no cold drafts.
Also, get the large chicks into a separate container, and start feeding those under-size chicks. This is critical if they are going to have any hope of developing right. However, keep the two groups where they can still all see each other or they will lose their group cohesion. This is important so they won't be in conflict when you put them back together once they move into the coop.
Gosh, how I would dearly love to re-write the rules on brooding chicks! Oh, that's right, I did!
See the article I wrote on brooding chicks outdoors. It's linked below under "articles by azygous". If you have any doubts chicks can handle cold temps, this should reassure you they can.