First, you should probably spend some time on
lowendmac.com. As a resource for paleoMacs, it can be useful. For example, it can tell you whether the Mac Plus can read 1.4MB floppies, or only 800KB floppies.
Second, please identify which OS version is on the Mac Plus.
Third, try formatting a floppy on the Plus, put a file on it, and insert the disk into the USB floppy drive. If it's unreadable, it could be:
1. The Plus's floppy drive is out of alignment.
2. 10.7 Lion doesn't read old HFS floppies.
#1 is best corrected by replacing the drive with one that's known good. Aligning floppy drives is not for the faint-hearted, and requires at least an oscilloscope and a specially recorded alignment floppy.
For #2, try moving the USB floppy to each of your Mac OS X machines. If the diskette formatted on the Plus doesn't work on any of them, then alignment is the likely problem. Stick it back in the Plus and see if it still works.
The SCSI port and an external SCSI CD drive is one possible pathway to the Plus. If you can find an external SCSI CD-ROM drive. It doesn't have to support burning discs, just plain old CD-ROM reading.
Then the pathway is to burn a CD-R (the once-only kind, not CD-RW) with the files you want moved from Mac OS X, using HFS format (not HFS+) or ISO 9660, and then move the CD across to the Plus.
I doubt you'll find an affordable SCSI adapter that can plug into any of your other Macs. You'd need a firewire-to-scsi adapter, or USB-to-scsi, and neither of those are cheap, AFAIK.
If you find an old SCSI-to-IDE adapter, that could work. You'd plug an ATA/IDE drive into a firewire-to-IDE adapter, format it as HFS (not HFS+) on your oldest Mac OS X machine. Copy files to it. Then unplug from Mac OS X, plug into SCSI-to-IDE adapter connected to Plus, and voila.
If this all seems like a remarkable amount of time and expense, then maybe that's why the previous owner of the Mac Plus unloaded the computer.