When you say "most", I think that means there are some that do sell Cortex parts in smaller packages. I saw someone mention a 2x2 mm package which certainly is a small part, most likely an 8 pin QFN with0.5 mm spacing.
If you look around a bit, I think you will find that the Cortex parts have become the new 8051 with an incredible diversity of sources, packages and capacities, not to mention prices. So there is virtually no longer a need for the 8 and 16 bit devices when you aren't running huge volumes.
I'm not sure why you link the number of pins needed to the type of CPU on the chip. I don't follow at all.
If you want to spend the time to become intimately familiar with two product families that's fine. But I think you will find that unless you are designing a product with anticipated production of 100's of thousands per year there will be no advantage of using an 8 or 16 bit MCU. So my advice is to find a single brand of Cortex that has a broad range of packages and capabilities with some very low cost devices and run with that. Of course make sure the supplier meets your requirements in terms of support and availability.
I still say simplify...