One, it uses ATA/EIDE for an interface, so unless your older PowerMac has that, forgetabout it. as for the early G4... why would you want to??? One, it won't look right. Two, it probably won't fit right. Finally, it might not work right. If you don't have a dvd decoder built into the video card (like you will not have on the older PowerMac models you listed) the drive will be virtually useless. All it will do is read cd's and data dvd's. You WON'T be able to view dvds/movies on the 7100 or 7500. I know for a fact that a 7100 is ONLY SCSI inside, and it appears that the 7500 series is too.
Don't waste your money, unless you feel like getting some hardware that will either be useless or not fully functional...