Yes absolutely possible. Ever notice that Macs have minimum system requirements? The Public Beta would have only been designed to support a select number of existing Macs at the time it was released only. Subsequent Macs might have supported it, but only if they also support OS 9.2 and no other version of OS X had yet been released. If your iMac was built after OS X 1.0 was released then no, the Public Beta absolutely won't work.