username/uid restrictions

[T. Horsnell]

'morning all,
I see from my new RHEL4 sysadmin guide that uid's up to 500 are
reserved for system use. When I first began setting up my
Unix userbase some 15 years ago, I unfortunately chose to start
at uid 100, so I now have to change the uid's of some 400 users,
36 million files, on a hundred or so boxes. OK, this is do-able
and should be more-or-less 'transparent to the user'.
(Since I'm going to make a change, should I start at 1000 and change
the lot? How reliable is the 500?)
The thing that concerns me more, is the plethora of reserved
usernames. There seems to be no rule to distinguish a reserved
username (presumably the list in the RHEL sysadmin guide is
going to grow) and its only a matter of time before some
newly allocated name collides with one which has been given to
a user. In fact, I cant find anywhere what the rules are for
usernames. Character-set? How many chars?
Pity that reserved names arent systematic in some way (like
always starting with sys_ or somesuch).

I'm not looking forward to the day when a new system-username
duplicates the username of one of the directors, which he has
had for the last 15 years, and which is also his email address 
held on mailing lists and institutions all over the place...

Any advice out there? 

Cheers,
Terry.