Ptolemy and ortelius are, in the long run, *not* meant to be used by toolserver users. Those boxes are explicitly separate. You can't run a production database when users are running all kinds of inefficient and uncoordinated queries on it. :) For now it doesn't matter, but keep this in mind.
Cassini is a toolserver, and managed by Wikimedia Germany. They do things differently than WMF, coordinate with them to see what works there.
I hope that we can have a joint on project on maps and use resources efficiently. For example, we might not have space for the full OSM database anywhere else then on ptolemy. However, I think we can find a way to provide production-level stability and stay within our resource base. Besides, I have no objections to having exactly the same production/monitoring features on cassini as well.
Okay just one question from my tool-author's perspective: Ortelius with a mapnik renderer will only need the PostGIS DB from Ptolemy, not the OSM-DB mirror. This Mirror will be used for Query-To-Map like features (and?) from the Toolserver.
So can't we go with a PostGIS-DB on Ptolemy and Cassini each, where Ptolemy is used from Ortelius for live rendering and Cassini uses it's own for customized render experiments with varying styles.
The OSM-DB however can be used directly from Cassini (r/o access of course). So what's lasting is the load balancing between the live PostGIS-DB and the OSM-DB used from the Toolservers, both residing on Ptolemy.
But I'm nearly sure I missed sth. :) Peter