You will have to use the Custom ones, the already existing ones do not even consider that they could generate there.
The whole Custom Ore Slot thing also applies to the worldgenerationnew.cfg for small Ores and Bedrock Ores. And for Bedrock Ores I can guarantee that those WILL generate if you use the Custom Slots.
Remember, do not use pre-existing Slots, those have a crapton of special cases attached to them in regards of optimization.