The IIfx will boot off 6.04 from a floppy. Mine was tested on from a hard drive with System 6.07 from an LC with a dead PSU and it booted just fine!

The IIfx is a bit more sensitive to signal bounce on the scsi port, so it has a terminator on the inside scsi port, leave it there. It you remove it, your scsi port will slow down as it tries to fllter out bounced signals echoing on the cable. This is normal on all SCSI systems, but for some reason the IIfx is more sensitive to it and the reason why that filter/terminator is there.

I'm glad you got it running. Let me check on that cap location for you.