The "too many open files" issue can be resolved by increasing ULIMIT_NUM_FILES in /etc/opt/scalix-tomcat/scalix-tomcat.conf.
As a rules of thumb it was said that one should calculate 20 open files per mailbox, but a heavy public folder usage will add additional files for SIS.
We have moved the SIS to a seperate Instance for systems with a large userbase (a couple of thousands mailboxes).
In general we only need to install the SIS webapp, but the installer is a bit keen when dealing with such special setups. It would be acceptable just to install a dummy instance, even with an empty messagestore and without license.
During the setup the IP of the real server should be used to whitelist the source address (look at the whilelist settings in sis.properties).
The InstallGuide PDF (p. 117) refers to another necessary point
When it is installed on a different machine, edit the following lines in the file /var/opt/scalix/<nn>/s/sys/general.cfg where nn varies with installation:
USRL_SIS_URL=sxidx://<FQDN:port>/%u
USRL_BB_SIS_URL=sxidx://<FQDN:port>/BB@<fqdn>
where FQDN is the host that the Scalix Search and Index Service is installed on, and the port is optional, but is almost always 80.
Once this it adjusted, the messagestore has to be reindexed. In theory it would be possible to move the indexes structure and do an ommodu to adjust the SIS-URL. But I would only do that if a full reindex is not an option due to the heavy load the index.browse are creating on the backend.
To reindex the messagestore, the "slow moving indexer" http://downloads.it25.de/scripts/misc/sx_slowindex could be used.