I've assumed that the memory for the Z400 and Z600 (all Version 2) workstations that we use would be cross compatible in memory, and so far that has been the case. I generally only use HP memory, usually bought used off eBay for these because HP "bins" their memory to be better than stock memory from the same suppliers, reportedly (and stated by HP).
The memory controller for these workstations is a 3 channel design, and Dan_WGBU (our favorite and very helpful HP engineer) has emphasized that 6 identical sticks is the way to get best performance out of the controller. I believe Dan used to have a souped up Z400, and maybe still does. He seems to know quite a bit of details on that box. He has posted recently on 8GB memory sticks for the Z620, about registered versus unregistered and single rank versus more than one rank, and the power draw of different 8GB sticks, and that what is available now in single rank was not available back during the HP certification process so that what you might not be able to do back then you might be able to do now with single rank and not loose memory speed. You may wish to search him out about your question on this, because the RAM changed between those two generations (ZX00 vx ZX20), but the principles may be the same.
Turns out that the official HP specs for "supported" does not always mean that only those specs will work. The X5690 was never in the official specs or supported, but it works great in the Version 2 Z400, and supposedly also in the Version 2 Z600. There are posts in here about running 48GB RAM in the Z400, and you can find them.
I'm running 6 x 4GB max in our Z400s/Z600s, and I'm using the following HP part numbers for eBay searching:
500210-071
500210-571
500210-572
661524-001
Don't go buying it all up!
Finally, all the variations in RAM can drive you nuts..... part of why I'm doing what I am with RAM is to avoid that because if its from HP with the right part number on it I know it will work.