Mixed RAM in iMac 27-inch 2020 i5
Solution 1:
I would use the mixed ram. When using mixed speeds, your computer will under clock the higher speed cards to match the lower speed cards so they all have the same speed, but since the speeds are so close together (2666 MHz vs. 2400MHz) you wouldn't notice a difference.
An extra 8GB ram is totally worth the 266 MHz speed difference.