Question about Crucial Mx300

In principle, yes, you can use them, they fit the connections. However, the Caddys are suitable for 3.5 inch SSD/HDD. You may have received an adapter for this. However, the SSD is neither on the QNAP compatibility list nor does it make sense to install it. Because the maximum throughput of this small NAS is only about 220mb/sec anyway. If I were you, I would install two normal sata HDDs. Which are suitable for 24/7 operation.