On AMD platforms, you can use 4 DIMMs at a speed cost (DDR5-3600 instead of 5200, stock frequency). Check your mobo & CPU manuals/datasheet to be sure.

Example: 7900X https://www.amd.com/en...

In theory, yes, but I have already experienced myself that two combined identical kits caused stability problems. If you want to be on the safe side, I have an alternative: Kingston Fury Beast (4 x 32GB, 5600 MHz, DDR5-RAM, DIMM)
You just have to be able to live with a slower clock and CL40.
Greetings