The directory formats differ slightly between System 6 and System 7 (more so on System 7.5 than on 7.1). Even though you can format a 800K Disk on System 7, it will be slightly off and you will, at minimum, rebuild the desktop on those floppy disks with System 6  to get them working. It would be easier to format the disks on System 6 with the SE\30 or Mac Plus and then copy files to it from the SE\30.

But in this day and age, you should take apart both floppy drives to clean and re-lube them.