[Exim] Exim and user account...

What is so simple for you, is a real problem for me...

I would like to run a mail-solution based on exim (for more than 300 users).
I installed exim, qpopper, cyrus-imad and squirrelmail (for webmail).
All seem to run correctly, but i have problem to creat users compatible with
the 3 servers...

I have some questions...

1. By default (if i have good understand) exim create one user account (smtp
account)
for each unix user who receive mail on the server?
2. So, is it secure to create a unix-account for each mail user?
3. Can i use the cmd "adduser -g mail -p password -s /bin/false -e 2002-12-01
-m user1" to do that?
In fact, is it a good decision to put this user in the mail group (the
exim's group),
and is it possible to protect against shell login (do exim run for this
user in this case)?
4. I have problems with users names format. Can i use a "smith_P" login? Is
this login format compatible
with exim? I prefere to use format like "smith.p", but it seem that
cyrus-impad don't accept
mailboxes with a point in the mailboxe name (the "cm user.smith.p" don't run
in the cyradm
program...)