PC floppies of that vintage were 720KB, not 800KB. I doubt the Plus' floppy drive will be able to read them, but it's worth a shot.

When I upgraded my SE to System 7.1 last year, it was able to read formatted DOS disks, but they weren't 720KB, just 1.4MB.

In addition, I would refrain from running 7.5 on a Plus. You're really going to tax the 68k processor. If you want to use System 7, install 7.1 max.