Does the MacMini Server require Mountain Lion in order to install CS6 and use as a workstation?

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If you are going to buy the computer now, the Mac mini Server will come with OS X Mountain Lion Server, so you will have to use it. You will be able to use it as a workstation (OS X Server is just OS X Mountain Lion but with the Server app), but Adobe CS6 doesn't run properly in computers with an integrated card, like the Mac mini. This doesn't mind that you won't be able to use Adobe CS6 in your Mac mini, but it won't run as good as on another computer

A iMac will be much better for CS6. Just for information, the Mac mini and the Mac mini Server have got the same graphic card, and if you can afford iMacs, much better

To expand a bit on what mende1 said, the iMac will be a much better option. The Mac Mini has only onboard graphics, while the iMac supports discrete graphics cards. This makes a big difference with Adobe software. CS6 can use the graphics processor to speed up operations (I can't remember whether that is turned on by default in Photoshop and Illustrator...that's always the first thing I configure when installing.) For small files and minor editing, there probably wouldn't be much difference between a Mini and an iMac, but when you get to complex files with many layers, blending modes, and filters, the difference in performance will be huge. You should also max out the memory if your designers are working with big files; a PSD file will typically occupy about 5 times the amount of memory the file takes up on disk. Similarly, AI and INDD files also benefit from having plenty of breathing room. It's also a good idea to have two physically separate hard drives. Adobe software uses its own swap files and assigning those files to a drive that's physically separated from the drive containing the OS and the applications will also speed up the system.