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System user names, uids, and gids

Debian uses some long (more than 8 chars) user and group names which I
don't particularly like, e.g.


This is annoying with ps(1) which abbreviates these names.  For more
than 20 years I have always limited user names to 8 chars on all my
Unix and Unix-like systems.  Can I rename Debian's user names without
problems or will they be re-created by apt the next time I install or
upgrade an affected package?

Also, I wonder why I have user names like Debian-exim in my
/etc/passwd although I haven't installed exim.  It was part of the
initial install but I have removed it afterwards.  Shouldn't the
package management remove those user entries, too?  May I remove it by

Next, there are many names which are both, a user name and a group
name.  However, I don't like that many of these don't have the same
numeric uid and gid.  Are these IDs fixed in Debian or may I renumber
them to my liking?