Which capacity ram do you really need for your 2020 iMac?
is mixing the ram capacity okay? I have tested to see the results.

Computer used in test:
iMac specification: 27inch/3.8Ghz Core i7(8 core)/AMD Radeon Pro 5500XT 8GB/2020
MacBook Pro Specification: 15inch/2.3Ghz Core i9(8 Core)/AMD Radeon Pro 560X/2019

RAM used:
8GB: 2x4GB
24GB: 2x4GB & 2x8GB
32GB: 2x16GB
40GB: 2x4GB & 2x16GB