A USB floppy drive should work - it works on my Retina MacBook Pro running macOS High Sierra just fine!
A USB floppy drive should be able to read HD (1.4M / 2.0M) floppy disks just fine, but will not be able to read older DD (800k / 720k / 1.0M) floppy disks.
Note that you will have best results using an MS-DOS formatted (FAT) floppy disk, rather than a Macintosh formatted (HFS) disk, but that would require your older Macs having PC Exchange or similar Extension active.