* can we set up multiple Solr home directories within the same Solr instance? (I want to use the same Tomcat Solr instance to support indexing and searching over multiple independent indexes.)
yes. using JNDI you can configure multiple instances of Solr each with a seperate solr home. the tomcat page hs more info on configuring solr home with JNDI in tomcat...
http://wiki.apache.org/solr/SolrTomcat
* If so, say I have some customized Solr plugins, ie., jar files, do I have to add them to each Solr home's lib directory? ( It feels it's a bit redundant to add them multiple times for the same Solr instance. )
you could, in theory, put plugins at a "higher level" classloader used by tomcat, but that would require loading most of hte solr/lucne code in the sam classloader -- i don't recommend that approach, and i won't give any advice on how to do it -- because classloader nightmares are bad enough when you understand them in detail.
i would recommend that even if you don't want to make multiple copies of the jars, you still use a seperate lib directory for each solr home and symlink each jar ... each solr instance will have it's own custom classloader for dealing with plugins, and this approach will help protect you from the possibility of "bad" code in any of your plugins causing one instance of solr to affect another (ie: synchronization blocks, singletons, global variables, etc...)
-Hoss
