Thanks for your questions!
Before answer your questions I need to confirm with your question, did you mean that you want to implement the labels are not allow overlapping for several layers? If that, first I need to tell you the overlapping property is just for the labels of one layer, so the overlapping property just affects to all labels on one layer, if you have many layers with labels, we cannot ensure all labels on all layers are not overlapped. The overlapping property is just for one layer's labels.
So to be implemented your requirements, I suggest you to do:
1, When initializing the layers, please void the overlap issue as possible.
2, Set the property of text style for each layer according to different zoomlevels.
I think the methods can help you to avoid the overlap issues for all labels on all layers as possible,
Any more questions please let me know,