You can keep all of the old RSLogix 5000 versions installed on your machine with Studio 5000. However, since you'll be running at least Windows 7 (minimum requirement of Studio 5000), you might have issues installing anything older than version 16 or so. For the really old firmwares, I would put them on an XP virtual machine.
The name change is confusing, so it's important to remember that Studio 5000 Logix Designer is no different than RSLogix 5000. It is the same software, just with a different name after version 20. Treat it just as you would any other version of RSLogix 5000, because that's literally what it is. The only new thing is that at some point in the future (Rockwell is saying 'soon', but they've been saying 'soon' since 2011) a new View Designer module will be added to Studio 5000 in order to program the new Panelview 5000 terminals. Supposedly this is coming in version 27, but again, this will not affect your old versions of software.