Do you mind me asking how they do without the heat at 10-14 days? I'm not too worried about my bigger girls hurting them but am worried that because if the run size they would get lost and vet too cold.
They still have heat available at 10-14 days, but they usually don't choose to use it. I have a mama heating pad in the brooder and the chicks can go in and out as they please. Generally they're out, but if they're tired and want to sleep, they'll return to the heating pad for nap time.
I don't fully give them access to the adult area (total 525 sq ft) until around 14 days, which is when I put in the panic openings and chase them around to ensure they know how to duck in if pursued. Haven't had a chick get "lost" at that age, even though I move the brooder around the run periodically.