It depends on whether your mac is capable of booting with 7.5. Many newer macs require a newer OS to boot.
If you have early model PPCs or pre-PPCs you should be fine. Any machine that came with 8 installed probably won't boot with 7.5.
What I don't understand is how you don't have the CD-ROM or startup disk for the OS that came with your computer? If you've gotten it used, then the seller should have given you these disks. You can order older versions of the system software from Apple, and in other cases from 3rd party resellers.
Good luck. Vive la Macintosh!
[This message has been edited by bluedog (edited 02-02-2001).]