Deleting unnecessary users/groups
 

In the /etc/passwd and /etc/group files there are 20+ entries, most of which I've never seen or used -- which ones can I safely delete?

If you created them yourself, you can delete them. Otherwise, some software wants them and you ought not to remove them.

Maybe. I dunno, it seems insecure to have a bunch of "extra" users (news, uucp, adm, etc.) on a computer that are there for no apparent reason.

They're locked accounts anyway - no one can log in no matter what password they might know. They just exist to own certain files, and as sandboxes for certain "dangerous" services, which you don't want to risk running as root.