Forgive me if this has been posted... my browsing did not find the answer, so here is the question:
Can I install older versions of MODx on the MODx Cloud? Specifically I'm looking at version 2.1.3, but in theory, I'm wondering if I could install raw PHP sites, WordPress sites, etc...
You should be able to install any version, provided you have a current install to re-purpose. You will need the Database & FTP/SFTP information, afterwords the rest should be easy. Granted if you are looking to run something else your own server might be a better option.
Yes, the Cloud is really just shared hosting. I have the Vanilla forum installed on my Cloud. While you would only have the one database, that is not really such a bad thing. If you are running something in your site that needs more than one database, it shouldn't be on shared hosting in the first place.
You should be able to run mostly anything on Cloud, but do keep in mind that you'll have to import it manually, and some of the features (snapshots, one click upgrade, add admin user etc) may or may not work.