If I buy a mac mini with a fusion drive and add windows using boot camp, can I make the SSD function be dedicated to the windows partition?
Yes, but it will be at the cost of a serious hit to the overall performance of your Mac mini. You would be better off getting an external hard drive with a USB 3 or Firewire 800 interface and dedicating that to Windows. You can even get external SSDs. If you are concerned about maximum Windows performance, especially for gaming, you might be better off buying a cheap Windows computer. The graphics in the Mac mini do not lend themselves to gaming.
Additional information from Linc's post. Basically, only one additional partition can be added to a Fusion Drive and it resides on the HDD.
Do computers that come with a Fusion Drive support Boot Camp?
Yes. Use the Boot Camp Assistant to create a Windows partition and install Boot Camp. The Windows partition will exist on the hard disk drive, not the Flash drive, and is not part of Fusion Drive Logical Volume Group. 3TB Fusion Drive configurations need to update to OS X Mountain Lion v10.8.3 or later to install Windows 8. See iMac (27-inch, Late 2012): Boot Camp alert with 3T hard drive for more information.