Partly HP gives an answer http://h20195.www2.hp.com/v2/getpdf.aspx/4aa4-6106enw.pdf about Radeon WX 3100 which makes sense in requirements being close to Quadro 2000, but it says nothing on Radeon WX5100.
Can anyone tell me if WX3100 and WX5100 are compatible in power requirements?
Both the WX3100/WX5100 are based on the polaris architecture, the WX9100 is based on vega.
Knowing that the WX3100 correspond to a RX540 and the WX5100 is similar to a RX570.
Knowing that the polaris architecture is not power hungry at all, i would assume that the WX3100 would run without issues and without drawing too much current from the cpu.
Regarding the WX5100, i would say it's ok, but keep in mind you could reach the psu full throttle, diminishing his efficiency and life span.
I would rather suggest a 550Watt if paired with a WX5100, just to not stress the psu full load, leaving some space for the cpu current demand.
This if the rig run H24, for a more normal use, if the Z400 own a decent and branded psu, it should be fine.
Probably not what you'd want to hear, but WX3100 and WX5100 aren't the best to buy for Blender work. Bear in mind that Blender is heavily consumer-grade hardware biased, I have personally not seen any improvement between 1080Ti, Frontier Edition and WX9100 in terms of consumer vs professional cards. I will add though that AMD hardware will not lock or stutter your screen while rendering, whereas the 1080Ti, while admirably fast, ends up killing multi-tasking because of screen stutter. Even on consumer hardware (i.e. RX series) you can render while watching a stutter-free YouTube video or just keep on modelling.
You're actually far better off just getting a decent PSU for at least one of the workstations and getting ONE hefty card, I can almost guarantee that you'll be able to do more with it because 1) 2x WX5100 + 2x Quadro 2000 will not match the render speed of a single Frontier Edition in Gaming mode(if working in Cycles, no difference between Pro and Gaming in Radeon ProRender); 2) you WILL need the massive on-board storage, or HBCC at least, and a bit more RAM.
wimpzilla is correct, you will get PSU throttle, maybe even shutdowns and system instability because of under-supplied PSU, that is really important. An EVGA 850W PSU is about £100, worth saving your equipment and work for such a low price, considering the professional environment.
I do for a living what you're getting into, with Blender and GPU rendering, if you have any questions whatsoever, either ask here or you can find me on AMD's Red Team discord, which is easier to deal with: https://discord.gg/qb7mHwV