I think ISPConfig will work fine on SLES 10 as i think the paths to programs and config files in SLES are similar to OpenSuse. If you get an error that the installer does not support SLES, please make a backup of the file /etc/SuSE-release and replace it with the file from OpenSuSe 10:
SUSE LINUX 10.0 (i586)
VERSION = 10.0
After the ISPConfig installation you can restore the original SuSE-release file.