I turned up the roomrheater so it's much warmer. I'm going to put in a little plushie or feather duster in there near the plate. Now their coop is ready just not the run would it be a good idea to move the older ones to it in case some of them are outgrowing the brooder?
Do NOT leave her alone in the brooder! That's just asking for more issues. The first is that she'll continue chirping for company, and the added stress of not being able to see or hear other peeps will put an added stress on her already taxed system.
The second issue - and it's a big one - is that, once she's ready to join the others, she'll be seen as an intruder. She's likely to be bullied by the others, especially if they're older, bigger or stronger.
If you DO decide to put the others out, separate the peeps into two small groups - one in the coop and one in the brooder. That way, when the brooder babies go out to the coop, they'll have their own little group to snuggle and socialize with while the new pecking order gets settled. It's always easier to integrate two groups rather than single birds. And remember to integrate properly, just like you would a totally new group of chickens. They won't necessarily remember that they were babies together!