The 7500 has what is referred to as an OldWorld ROM. OldWorld ROM's don't have the ability to boot Linux using yaboot which the CD uses.
In order to install see the following copied from the install guide located at
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OldWorld ROM systems: BootX
This installer assumes 2 things: 1) That you have split your drive into 2
partitions or have 2 drives (one for Mac OS and another for Linux); and 2) That
you have already installed Mac OS on a partition or drive.
1. Reboot your computer into the newly installed Classic Mac OS.
2. Double-click on your startup drive and then the System Folder.
3. Insert the Yellow Dog Linux Install disk #1 into your CD-ROM drive.
Double click the CD icon.
4. Double-click on the folder images. Drag the ramdisk.image.gz to the
System Folder. Close the folder images.
5. Double-click on the folder boot and
then BootX_1.2.2.sit(which should
activate Stuffit Expander).
6. From the newly created BootX
1.2.2 folder, drag BootX App into the
Control Panels folder, • BootX
Extension into the Extension folder,
and Linux Kernels into the System
Folder.
7. Close the folder BootX App and now return your attention to the boot
folder that was created by Stuffit Expander. Drag vmlinux-2.4.20-8d and
vmlinux-2.4.20-8dBOOT from the bootfolder into the Linux Kernels folder
which now resides in the System Folder.
8. From the Apple Menu Items and the Control Panels, select BootX.
9. Select Options... and then “Use specified RAM disk” wherein you will
choose the ramdisk image that you previously copied from the CD to the
System Folder, where it now resides.
10. Select OK.
11. From the drop-down menu to the right of the word “kernel”, choose the
bottom of the two which ends in “BOOT”.
12. Press the button Linux. Your screen will temporarily freeze and then
take you into the Linux installer. If your computer does not boot into
Linux, and appears to be locked-up (give it a good half minute), force restart
your computer (CONTROL-APPLE-POWER) and when next to
come to this final step, first select the No video driver option
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