The upgrades offered here on iFixit for the 2017 21.5” and 27” Retina iMacs both have PC4-19200 modules.
According to everymac.com, that model supports up to 16GB of RAM.
Technically it should work. It’s sub-optimal to have mismatched modules, but more memory that doesn’t match is usually better than less memory that is matched. However, since obtaining another 8GB module is relatively inexpensive, while opening up your iMac is somewhat risky, does it really makes sense to go to all that trouble and only replace one module?
Yes, the processor model is not important. The 16GB maximum seems to be an artificial limitation set by Apple for marketing purposes.
You can install 2x 16GB for a total of 32GB.
Lately I’ve been installing Crucial MX500 SSDs and so far, so good.
If I remember correctly, I had the most difficulty with the power supply connectors. They require more force than most of the others. The most important thing is to pay close attention to which direction each connector must be pulled to disconnect it. Study the pictures closely and make sure you have the right tool for each step. Having an extra spudger is very handy.
Have you ever done anything like this before? Experience helps a lot because you learn to recognize the different types of connector. This informs not only the direction the cable should be pulled, but which part is the cable and which part is the socket.
If you’re not confident, it might be worth paying for the labor to have someone with experience do the job. If you ruin the logic board, it will cost a lot more than $240 to repair or replace.
Check to make sure the I/O ports are flush with the back of the case. It’s easy to end up with a small gap, and if you do, you won’t be able to fully insert USB-C connectors, and the Thunderbolt ports won’t work.
Take a look under Parts at the top of this page.
At step 60, when reinstalling the logic board, check to make sure the I/O ports are flush with the back of the case. It’s easy to end up with a small gap, and if you do, you won’t be able to fully insert USB-C connectors.