You can find release notes for all versions (not 100% sure, but at least since 4.0) and look there for supported supported source/destination/OS versions; just google for them. BTW this is not the case for downloads - you can download 5.5 and 6.0 from the site.
About the confusion caused by 'vCenter' - there used to be a converter extension for vCenter Server which has been discontinued (last version 4.2) and which had no 'standalone' in its name. Now there is only the standalone version. The old standalone versions after 3 are: 4.0.0, 4.0.1, 4.3, 5.0.1, and 5.1.2
HTH
Plamen