Macs and PCs (Running MacOS and Windows respectively) use different file systems (HPFS I think for Macs, but that looks wrong. And FAT on PCs). Macs can read FAT16 at least, and 3rd party programs can read Mac formatted disks on the PC. If you use a file format and file system that both systems understand, you shouldn't have any problems.
Yes, 250MB Zip drives can read 100MB disks.