For WinUAE, you don't need to use IDEFix as the emulator uses the host system's IDE controller to connect and communicate with the HDD, so it should be able to access a HDD as large as the host system's controller can handle. You only need IDEFix on a real Amiga when using its built in IDE port because of hardware limitations and its design originally only being for a max of 4GB HDD size. This is to prevent the interface getting confused by larger HDDs and thinking the space above 4GB is the start of the drive again and writing over the drive, corrupting the data.

Newer scsi.device drivers are patched to allow 8GB HDD sizes, and OS3.9 is supposed to naively support larger HDDs out of the box, but users have had mixed results with this. I personally always use IDEFix.

I can't answer your question about the guru's or crashes due to the system software as I've not had similar issues. 8000000E is definitely a memory error, but it could just mean the software did something that caused a memory conflict, not always just because of a ram fault.