I have one Mini with no music connected to an audio amplifier and three other Macs each with its own library connected to the network. I also have a file server, but the Mini only has a Wifi connection to the server and I've had problems getting that to work all the time.
IAC, first, in iTunes preferences, go to Advance>General and unselect "Keep iTunes music folder organised" and "Copy files to iTunes music folder". I assume you have already gone to Sharing in preferences and turned on "Look for shared libraries" and "Share my library".
Once you have unselected "Copy files..." you can just drag files from anywhere into iTunes. If you have more than two Macs, you will need to do these steps on each one that you would like to share. Assuming the Windows version of iTunes also has these preferences and you've turned them on, your Mini should see that library (I assume you will need to also look at you Sharing control panel to be sure you've got "Windows Sharing" turned on.)
I would advise against copying .xml files around because all the features you need are already built into the program and just have to be "turned on" correctly. In fact, I would let iTunes rebuild its .xml files, then reset the preferences I have described above and try again. HTH.