I think the PNY 256GB SDXC card will work in a Gen. 5.5, and I have that build ready to commence so I can confirm it later.
There is some conflicting information out there about which iPods suffer from LBA28 lockdown in their Apple firmware. You can still get around it with Rockbox according to Tarkan, however I think the Gen. 5.5 will work using the stock Apple firmware.
What definitely did not work in my own testing was an "early original" Gen. 6 Classic, even custom CF- SD adapter firmware from Tarkan did not overcome the LBA28 limitation, the Restore goes fine but the SDXC becomes corrupted after passing the 127.7GB limitation of LBA28.
What didn't make sense to me was why an older model Gen. 5 does not have the stupid Apple imposed LBA28 lockdown, compared to the slightly newer Gen. 6 and 6.5 which does have the LBA28 lockdown.
Logically you'd think the newer the model, the more likely it would have LBA48 firmware, because Apple was selling 160GB Classics that would have needed it. But the Classic was all over the road for a couple of years with the capacity they offered, it ranged from 80GB, to 120, to 160 depending on the exact vintage.