So I'm a newb to Linux, and from what I've read, knoppmyth installs are out of the box only safe behind a firewall. I'm running R5E50.

Well, I've set up some of the cool stuff like mythweb and webmin, and I'm going to want to be able to access mythweb from outside of my local network.

I know how to set it up, but I need a bit more info about the security side of things. So far, for webmin and mythweb I've deleted the default accounts, and replaced them with custom ones.

I know there are other logins though that are mythtv/mythtv as the login and password. These I would like to change. I've searched the forum, but I can't seem to find anything on the subject. Does anyone know how to change any accounts that are mythtv/mythtv to something else and not break it?

You can manually change the mythtv account's password with (as far as I can tell) no ill effects. Just use the passwd command.