According to the
Cloanto website you can use a USB serial cable but you will then need to get a DB9 to DB25 serial adapter and all should work fine. Just make sure you get the proper orientation of connectors - Male/Female.
Regarding the floppies, yes they will work in DD (double-density = 880kb= normal) mode. Back in the day we used to buy HD floppies because were cheaper than DD floppies and they worked fine on normal Amiga floppy drives.
