Try the heat lamp first..Chick height and remember the temp will drop over night at around 4 am....Tricky yet very easy to set.....;)......Chicks need to be warmer..Ducklings are so easy...:)
As long as the brooder stays the temp it should be? No need for the space heater...;)
I would keep the heat on and have the thermometer at Chick hight....You have it set up so the Chicks can move away from the heat if needed..:)......pasty butt is caused from the Chicks getting to cold....even raise the lamp up a bit till it is at the proper temp....
Best wishes....:)
Cheers!