Have a scsi card reader from MechWare, no need to update the ROMS it'll work just fine with the A590.
The rom upgrade for the A590 is for what HDD's are supported.
Quote - "Version 6 ROMs only allow hard drives up to 512MB in capacity. Version 6.6 and 7.0 ROMs have been reported to work successfully with drive sizes up to 9GB but you will need at least OS3.5 to use drives over 4GB"
I eventually upgraded to 7.0 ROM's so I could have the larger cf card :-)
I've had a problem though trying to get it to boot from the cf card on it's own with the pcmcia adapter. It's possible apparently but the method that is supposed to work doesn't work with my setup so can't fit the entire card reader inside the case.