Thus Disk Copy 6.3.3 is not appropriate for pre-System 7 disks. It will image 800KB HFS disks, but you can't convert the image into a real floppy on a Plus running System 6. For disks intended for use with System 6 and earlier, Disk Copy 4.2 or DART are better options.
For compressing files, I suggest Stuffit 1.5 for the System 6 and earlier era software. Stuffit 1.5 is a bit clunky to use, so for System 7 onwards, I always use a compression program that is compatible with Stuffit Expander 3.5. Stuffit Expander 3.5 is easy to find -- Apple shipped it on system install CDs -- and it is the lowest common denominator standard for System 7. Later versions of the Stuffit compression utilities "enhanced" the product, requiring a later version of the Expander. Some of the later versions of Expander do not work on 68K Macs, hence my preference for version 3.5 compatibility.