The general answer is "yes", but you need to be careful. The issue is not so much in mixing different sizes of RAM (I was doing that here for a while on a very similar motherboard), but with other properties of the RAM.
Your motherboard manual (you can download that from SuperMicro here if you don't have it) has a table showing the limitations. The table you want is on page 2-9. "1.5V RDIMM Population for the Motherboard w/5600 Processors Installed"
Your system has 3 channels per CPU socket, and two slots per channel. You presently have one slot in each channel occupied. Your existing RAM is 1333 MHz, so you are in the top row of the table. To use that RAM with two slots filled per channel will put you in row three of the table, which limits you to single-rank or dual-rank RAM. Your existing RAM is dual-rank (DR). If you go to quad-rank, your RAM speed will drop to 800 MHz (row 4).
I can't see from the spec sheet how many ranks the RAM you want to add has, so there is a question there, esp since denser DIMMs tend to have more ranks. Even the 16 GB modules by themselves, if quad-rank, would slow your RAM speeds to 1066 MHz. Numerous tests have shown that memory clock rate is nowhere near as important as it would seem to be for performance, but if you're after ultimate performance, it's something to think about.