Is there a single server I can use to store/manage repositories which could then be accessed by the common/modern open-source (D)VCSs such as SVN, Git, Mercurial, bzr, etc and still keep a good level of compatibility with all of them?
We have a large number of users on SVN, some on Git, and a couple using mercurial but none are using any overly advanced features at the moment, but hitting 100% support for each VCS isn’t a priority. Coping with them all with one server is more important, especially if users can check in via SVN while another checks out via Git.
If your main users are git and SVN then your can simply use an SVN server. Git supports checkout of an SVN server and checking changes back into the server using git-svn. You can probably get similar functionality with the other distributed SCM tools (a quick google for it with mercurial shows there is some third-party support for such features while native support is being considered).
For now, that is probably the simplest solution – keep an SVN server and supply instructions for your users to checkout the repository with their favoured tool.