Is there a way to have multiple versions of messenger at the same time?
Yep, with the exception of mixing the 2009 and above versions, all you have to do is put each in its own folder and you can run the executables manually at your leisure.
Note however, that if you want Plus! to work with each, you have to do some more wrangling, which is a little more complicated. However, if you install each Messenger+Plus as per normal and copy over the Messenger folder from each into a separate location, in most cases, that should give you a working setup as above.
For example, right now I have 7.5, 8.1, 8.5, and 2012 (from before, doesn’t work of course) all installed and usable.