To clarify: 800k floppies were used on even older Macs than the 636. They're different to PC 720k floppies and not interchangeable. The USB floppy drives will only read and write 1.44MB floppies.

The 636 can read both of them, so you'll have no problems using 1.44MB floppies to transfer between it and the iMac. And if you ever do get an older Mac in the future that only reads 800k floppies, the 636 can make them for you