iMac 2019: Can I mix the old modules with the new ones when upgrading RAM? I've found a similar question but it's about the 2016 iMac, so I want to double check for the current (2019) model.

Can I mix new RAM (Crucial CT2K16G4SFD8266) with Apple's stock RAM? I am planning to buy an additional 32GB of memory, so it'd be a total of 40GB - is that even a number the 6 core iMac can handle?
Yes, you can mix them. There are 4 slots, so you can keep the Apple RAM, even if it's in 2 slots.

According to the reliable EveryMac, your Mac can take up to 128Gb !!