2012 iMac RAM Upgrade I've just ordered the 27" iMac, which has user-upgradable RAM slots.

There are four slots, and the default configuration that ships is two 4GB modules, and two empty spaces.

Can I fill the other two modules with 8GB chips and bring the total to 24GB?  Or, is it not possible for me to mix two 4GB modules with two 8GB modules.
Yes, you can and it's quite common practice.

https://support.apple.com/en-us/HT201191

One concern might be how the memory channels are laid out, as it's generally better to have symmetrical RAM configurations to balance dual-channel access. (so you'd want each channel to be 4+8)