Will it modify the way joomla authenticate customers basically give a custom user groups within the Joomla 1.5.15? Will it allow it to be less secure?

I am likely to give a custom group only at that table: jos_core_acl_aro_groups

http://docs.joomla.org/Custom_user_groups

Someone explained when I give a new user group and also the group_id is more than 25 (this is actually the ID from the Super Administrator) that new group will have a similar access because the super admin within the default joomla core files without altering anything only the additional user group. Is true?

Don't you've got a local instsall of Joomla to do this?

Anyways, it isn't true since it can not be, why must the audience_id define the access privileges? That might be a dreadful ACL implementation. But please check it out, before you decide to really utilize it live.

In Joomla 1.5 you may make user groups but they're going to have among the existing role designs. In Joomla 1.6 (alpha) you will see full versatility in determining your groups, roles and granular possession configurations for every article, module, etc.

You will find several extension you'll find at: http://extensions.joomla.org/extensions/access-a-security

Which boost the core ACL functions. Provide them with a glance and you will most likely discover the solution without hacking the core files.