Hi
IIRC the minimum OS for that particlur MacMini model (Snow Leopard Server pre-installed) was 10.6.3 (Build 10D2235):
http://support.apple.com/kb/HT1159
It's the different build versions for the different OSes you have to be mindful of:
http://support.apple.com/kb/HT1633#retail
If the DVD you're using is the same OS version but different build, it won't work (the reason for why you're seeing the no entry sign' regardless of how you try and provide the installer to that hardware.
". . . or can a server not install SL normal?"
The only thing that makes that particular unit a 'server' is the Server OS that came pre-installed on it. There is nothing in the hardware itself that would make it a server as such. In fact it has a lot of what most people would regard server must haves, missing.
Perhaps the 'best thing' you can do is to install the final release of SL 10.6.3 Build 10D575 on any qualifying Mac and update it to 10.6.8. Target Disk Mode the MacMini to that Mac, reboot that Mac from the Installer DVD and use Disk Utility to restore the Installed 10.6.8 onto the MacMini.
HTH?
Tony