Error : unknown file system while trying to install Debian on my Mac mini
I was able to reach the point in the installation process where I needed to create partitions to install Debian. This partitioning was part of the Debian installer. I did not encounter the same problems you have reported. 

I am not sure why you are using rEFIt and/or rEFInd. I did not need to.

Some of the steps I took:


I burned the file "debian-mac-8.6.0-amd64-netinst.iso" to a RW DVD
using the Disk Utility application. The OS X was Yosemite. I assume
Snow Leopard would work the same.
I did not need rEFIt or rEFInd to start the installation. I just
restarted the Mac and held down the option key to invoke the Startup
Manager. Next, I inserted the DVD and waited for the DVD icon labeled
"EFI boot" to appear. Finally I select this icon to boot from.
I choose the "Graphical install" option when prompted.
My connection to the internet is hardwired, so I did not have to
configure wireless settings.
The installer did not have the software for Broadcom wireless
hardware in my Mac. I instructed the installer to bypass this
problem.


Perhaps you could modify your question to include the output from the following Terminal application commands. These commands will not change your Mac. The commands just give me a better understanding of how your drive is partitioned.

diskutil list
sudo gpt -r show /dev/disk0
sudo fdisk /dev/disk0