In OS X v10.8, most of the APIs in the Carbon Core framework are deprecated. In many cases, there are alternative APIs you can use, such as APIs in the Core Foundation, Foundation, GCD (Grand Central Dispatch), and Disk Arbitration frameworks.

I don't have ANY issues with Snow Leopard on my Mac mini 2011 quad server.
 
I am not convinced that using a retail disk is a good idea - when I have tried that in the past with other Macs it never got all the drivers and never added all of them after a Software Update either!
 
I used 10.6.8 (10K549) from my MacBook Pro (Early 2011), as it has exactly the same hardware as a quad-core mini.  No boot problems with this, all drivers working good.  I actually used carbon copy cloner to an SD card at first, and booted off that before I restored it to the mini server's second hard drive.
 
BUT there are serious performance issues, as Snow Leopard doesn't seem to speedstep the processor correctly with a machine identifying itself as being a Macmini5,x, so any of:
Macmini5,1 - dual core with Intel HD3000 graphics
Macmini5,2 - dual core with AMD graphics
Macmini5,3 - quad core i7 (server)
 
Basically to fix this you need to modify a kernel extension, and update the caches - You need access to a copy of Lion for this - I used the copy of Lion which came preinstalled on the second hard drive.
 
USING the Lion partition (either using Finder > Go to folder…, or by using the Terminal) you need to go to:
/System/Library/Extensions/IOPlatformPluginFamily.kext/Contents/PlugIns/ACPI_SMC_PlatformPlugin.kext/Contents/Resources/
 
FIND Macmini5_1.plist, Macmini5_2.plist or Macmini5_3.plist (depending on your Mac mini 2011 model, or copy all of them if you aren't sure).
 
COPY these back to the the same location on your Snow Leopard partition.
 
REMEMBER, depending on how you copy the plist file(s) you may have to change ownership of the file(s). The plist file should be owned by root and the wheel group on Snow Leopard.
Using Terminal and Snow Leopard partition:
sudo su -
[enter your admin password]
cd /Sy*/Li*/Ex*/IOPla*/Con*/Pl*/AC*/Con*/Res*/
chown root:wheel Macmini5*
 
FINALLY, update the kernel caches using Terminal by typing:
kextcache -system-prelinked-kernel
kextcache -system-caches
 
THEN, reboot your Mac mini!!

After we finished with the amenities and reminiscences, we got to the purpose of the meeting. Steve wanted Compaq to offer the Apple operating system on its PC line, adding to the Microsoft OS that had always been our sole OS. At the time, Compaq was the world's largest manufacturer of PCs. Our adopting the Apple OS would be seen as a feather in Apple's cap (and a pretty visible slap at Microsoft).
The catching up with Steve was fun, the food was great, but the OS idea never gained traction. Upon further analysis, it didn't make sense for either Compaq or Apple. Compaq wasn't about to declare war on Microsoft, our partner from our birth in 1982, and Steve had second thoughts about licensing their crown jewels.

“I inherited the competitive iPhone OS project from Jon Rubenstein and Steve Sakoman when they left Apple. I quickly shuttered the project after assessing that a modified Mac OS was the right platform to build the iPhone upon. It was clear that to create the best smartphone product possible, we needed to leverage the decades of technology, tools and resources invested in Mac OS while avoiding the unnecessary competition of dueling projects.”