Voltages mostly affect PC/MCP.
Gen3 can go up to 1000v, which may be a little high if you go into a Photonis tube with a low halo ( ES Field Intensity is the issue )
Most run at around 800v ( early Gen3, Gen2 ) to 600v ( Thin Film ) to around 200v ( Filmless ) -
MCP voltage can be set with gain.
Screen cutoff voltage can be set fairly well with ABC.
So, I think Photonis should probably run best at around 600v - So a thin film tube should be OK and autogated ones would probably work OK too -
Now.. I haven't tested any of that, but generally, as long as you're not getting problems, and any problems will likely show visually, then it's OK to use a Gen3 psu - though you might suffer a little loss of image quality -
Basically, these things just work of electric field strength afterall... If it's not arcing, it's probably OK.
Regards
David