Multiple Sites with Subdomains, Common Code Base and Log in

Because Drupal 6 takes incremental performance hits with each added module I want to break up my site into one main site e.g. www.mysite.com and several subdomains of it e.g. shop.mysite.com, forums.mysite.com, etc, where the main site and its subdomains have only the needed modules and have a common loggin among them all (once logged into any them the user is automatically logged into the rest.) In addition I want the subdomain module to function so that organic groups have subdomains of the site forums.mysite.com, e.g. group1.forums.mysite.com, group2.forums.mysite.com, etc.

I do not want very much content to be shared among the sites. Only things such as user profiles and log in data would be.

The database is common to all the sites.

Because the subdomain sites would have access to the main site's cookie I do not expect to need the singlesignon module. I expected that if I set up the table prefixes for the main site and each of the subdomain sites all I would have to do is install Drupal 6 for the main site mysite.com, log off the main site, and install the subdomain sites. But when I do this I get a lot of errors when a subdomain site is installed. Some of the errors are to be expected because they are errors caused by the shared tables already existing (they were installed when the main site was.) But there are other errors that make the subdomain sites not useful.

I realize that it is about certainty that doing this will require hacking Drupal core. I am willing to live with that.

A reply from anyone who knows how to do this is very much appreciated.