Old school Mac question- installing older OS

At work our store network is on Quadra 650s. They use 7.5.

We just picked up a few more and they’ve got 7.6 on them, which won’t run all of our software. If we have a system 7.5 cd, how can we install 7.5 on these computers to get rid of 7.6? Nothing on the hard drive is important.

And since I’ve just been informed that handy dandy Hard Disk Utility is basically all I need, I feel quite silly.

Oy, I am so smart.