Do you actually have need for more than 16GB of RAM currently? For things like gaming and the vast majority of applications, 16GB should be plenty for at least the next couple years, if not longer.
If you were to put that slower memory in, it would slow your RAM speeds down to 2133 MHz, which may noticeably hurt your CPU performance, and if you don't actually have some usage scenario that could make use of more than 16GB (like certain video editing tasks), the extra sticks will remain unused, providing no benefit.
As such, it would probably be a better option to sell the slower RAM, and buy more faster RAM once you eventually have need for it.