As a Creative Cloud subscriber, may I get CS6 InDesign for free?
You can download it and install it, but it isn't free. Access and use of it is included in your subscription.
Note that if you want double-clicking on an InDesign document to invoke InDesign 9 instead of InDesign 8, uninstall InDesign 9 followed by the installation of InDesign 8 followed by the re-installation of InDesign 9. Clear as mud?
Actually, that doesn't necessarily work on Windows since InDesign installs its own extension handler which typically overrides the registry's association of file type with an executable. The more newly installed extension handler wins.
@dov. it works fine, we have 13 pc's running that way, but indeed the newest app wins. You have to change it after the latest install, and once or twice you have to fiddle a bit in the registry to get it up an working. But, the best way to avoid problems is to leave it as is, the newest app is the default one.