Here's how I think it works ... could be wrong.
(Note: I just did the 'install all' which laid the legacy content down anew, in addition to keeping my old content).
If you don't download the individual/legacy installers ... it will determine whether you have Dim/Rap from Registry entries and 'copy' the content to the RapPro Multisamples directory (and leave the old stuff in place too).
In my install it didn't allow me to change the 'Patch and Presets'(progs) dir - greyed out and pointing to the old location. But this may just be a restriction on the 'install all'.
Expansion pack content was not copied ... I've manually created links so as not to have duplicates (I want to keep Dim/Rap), which is working well.
So apparently, I should be able to play Dimension Pro sounds through Rapture Pro, and I can delete/uninstall the 1.5 version and install Legacy Content Packs 1-3, or should I leave everything the way it was and not install Legacy Content Packs?
You can do either ...
With (a) - you would have the content but longer download/install times for the legacy content.
With (b) - that should handle the migration (copy) of at least mutisamples. This option would at least let you assess the situation after install (/prior to Dim/Rap uninstall) ... and especially wrt your 3rd party expansions. 30GB is enough space to do that.
I'd go (b).