Architecture of subdomains for different sections/departments sharing maximum number of tables

We have a main PUBLIC website in the drupal 4.7b5 root.
mainsite.org

- We have the Editorial sections sharing the tables
$db_prefix = array(
'default' => 'EXTRA_',
'users' => '',
'sessions' => '',
'role' => '',
'authmap' => '',
'sequences' => '',
);
and the Drupal files in the subdomain extra.mainsite.org

This same we have done for the administration section
admin.mainsite.org
and for the Office section
office.mainsite.org

Al are running +/- well
(modules in subdomains: ACIDgallery, events, groups)

Questions:
1.- Some time moving across the website They request Login?
What is the optimum time for cleaning the sessions table DBmaintenance
2.- There are any Good/Best Practices Guide for Drupal administrators write in the documentation
3.- There are more table that we can share (at the moment there are 228 for the 4 sites).
4.- How to share/configure the search if there are restricted areas. Each subdomain which one cron indexing?
5.- Can we add a D4.7.B6 in a new subdomain?