I have a CRS326-24S+2Q+RM connected to a Mellanox ConnectX-3 (two QSFP+ ports) via a copper cable, specifically a Q+DA0001.

I was looking to get another Q+DA0001 to connect the other pair of 40Gbps ports. The problem is, my go-to shop only has XQ+DA0001 available, which is a 40G/100G QSFP28 cable.

Can I use a QSFP28 DAC in place of a QSFP+ DAC? I've read they're basically the same form factor, but I'm not sure if it will actually work.

QSFP28 vs QSFP+ is actually SFP28 vs SFP+
SFP+ is 10G max, QSFP28 is 25G max.
Both are electrically and mechanically compatible, but not all (Q)SFP28 ports support 10G.

So technically it should work in your case.

For cable, also have a look at fs.com, where you can get active optical DACs for less than most copper passive DACs and is much more reliable.
https://www.fs.com/uk/c/dac-aoc-aec-cab ... 7?70=21671
(not affiliated with them, just a happy customer)

I just found out Mikrotik has a chart for this exact question. It's over at https://wiki.mikrotik.com/wiki/MikroTik ... patibility

In my case, the answer is yes.

Not sure about the Mellanox though, but I'm willing to bet it's a yes as well.