The broody hatched chicks may do better with a heat plate as they want to go under something to get warm...probably why they were trying to get under the older chicks.
Piling can happen with bator chicks too if the heated area is not large enough to accommodate them all, sometimes then it's...
I think you may have answered your own question.
How old were the younger chicks when you removed them from mama?
You might need multiple different heat sources if you want to keep different aged chicks together.