I have looked on the webmin site, old virtualmin site, and this virtualmin site and have not been able to find a list of supported servers for the GPL version nor the professional version. Among my main concerns was Dovecot. Do either of the virtualmin versions have support for any IMAP server such as Dovecot, or is VPopMail the only option?

I'm not sure about GPL, but Dovecot is installed and setup VM Pro

Dovecot is the preferred and recommended IMAP/POP server for both GPL and Professional. There's even a pretty comprehensive configuration module in Webmin for it.