I don't know about System 7.5, but I have a PowerMac 6500/300 that came with both System 7.6.1 and an internal ZIP drive. I think you do need to download some drivers from Iomega to get it to work however. I do know that when I tried a clean install of OS 8 some years back, I couldn't use the ZIP drive. I think I also had to upgrade the ZIP drivers at some point, but I can't remember if it was when I upgraded to OS 8, applied the 8.1 patch, or upgraded to 8.5 and applied the 8.6 patch.
All I know is that you can use both the internal and the external ZIP drives with 7.6.1 if you download a driver from Iomega. Weather or not you can still do so is another issue. I could extract them from the RestoreCD, but it wouldn't have an installer.