In an effort to help others who find their way to this post and are wondering if they need to use both Overviews and Pyramids in their mosaic dataset, I can share what the deciding factor was for us:
When building more complex mosaic datasets, especially where you will be taking advantage of the mosaic methods and on-the-fly processing, then it can be advantageous to build pyramids on the source rasters and to build overviews only where they are needed, such as when
Our orthophotography tiffs DO overlap, and therefore need to take advantage of mosaic methods. Because of this help document, I decided to use BOTH pyramids and Overviews.
I hope this helps someone else, too!