Sounds good.
80GB for Windows 7 should be enough.
I have ISO files of all the CD's of games I want to play - I have dug out an unused 4GB SD card from a drawer, so I can use that to copy the ISO files to Windows 7, mount the ISO files with a virtual CD mounter, and install the games.
I will format that card to FAT32, as from what I can tell, AROS only has read support for NTFS, and it might be handy to have read/write access from Windows 7 and AROS.
The ZG5 has an SD card slot, so might as well make use of it.
I am guessing that if I shrink the Windows 7 partition after I factory reset it, AROS will install to the unused space on the disk and leave Windows 7 alone.
I am also guessing that the grub installation performed by AROS will see the Windows 7 installation, and add that as a boot option.