[Typo3] extending the pages table and more

[Antonio Reher]

Hello Typo3 users:

I'm an intermediate Typo3 user. I normally use TemplaVoila and have Typo3 v. 3.8 installed, and I have created several simple extensions. Everything seems to be working fine but I have several concepts that need to be cleared up:

We are planning on redeveloping or site with Typo3 (http://www.sonoratec.biz). In the clients section we'd like to divide them into categories, so that via a plugin we can insert a selector box and bring up a menu with relevant clients. After thinking a while I have figured out that this could be done in two ways:
1. making subpages for each of the categories and inserting clients into each one. This would be the easy way but the problem is that some clients may be in more than one category. The only way of overcoming this would be to also create shortcuts in the other categories.
2. via an extension, creating a category table and then extend the pages table with a m-n database relation. This is for me the better solution, but there is one problem: the selector box for the database relation would appear in every single page I create. Is there any way of restricting it to only, say, the pages under clients?

Are there any other suggestions?

There is one other matter: we're planning on making a login box for clients and then making a documentation page. When a particular user visits it, he/she will only see some of the docs. How can i hide content on a page to some users? 

Thank you everyone!

Antonio