It is possible to download software and make floppy disks of software for Macs using a PC, but it needs care to avoid damage to the resource forks of Apple files.
Even when you use Macs alone, it is preferable to use StuffIt Expander (5 or) 5.5 for older Mac software, and nothing later than Expander 7.0.3 for OS 9 software. StuffIt 11 has no special role to play. Mac OS X looks after itself.
If you are getting .sea where you expect .img, you can always use DiskCopy 6.3.3 to make floppies from the self-extracting archives.
Even when you use Macs alone, it is preferable to use StuffIt Expander (5 or) 5.5 for older Mac software, and nothing later than Expander 7.0.3 for OS 9 software. StuffIt 11 has no special role to play. Mac OS X looks after itself.
If you are getting .sea where you expect .img, you can always use DiskCopy 6.3.3 to make floppies from the self-extracting archives.