Nope, 10.6 will not work on any G5, it's IntelMac only.
Don't use Tiger either to try to change Passwords on 10.5.x
Do you remember if you had this set?
Firmware password protection in Mac OS X ...
It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option, and Shift, as well as booting from anything but the Hard Drive.
Are you using Filevault?
Reset OS X Password Without an OS X CD...
Starts up like the first time you buy a new Mac, but after filling in all that info again, you should have access to the computer and the other Users & files will still be there... give the new User a different name than an existing one.