it sounds to me as though you're going about this correctly--bringing your legacy data into a separate index is a good call. one thing to make sure you understand is how Splunk archives/freezes data, described here:
http://docs.splunk.com/Documentation/Splunk/5.0/Indexer/Setaretirementandarchivingpolicy
in terms of the source types, if your data is of a standard format (AD, OS, and network devices are all pretty standard), Splunk should do the right thing by default. read more about that here:
http://docs.splunk.com/Documentation/Splunk/5.0/Data/Whysourcetypesmatter
http://docs.splunk.com/Documentation/Splunk/5.0/Data/Listofpretrainedsourcetypes
hope this is useful.
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