/etc/apt/apt.conf
/etc/apt/preferences
I would like known if this configuration is sufficiently secure or if is better change the priority on some repository.

Ok,i'm confused.
You are running Stable,and i assume your sources.list reflects this,why are you pinning individual repositories that should be pointing to Lenny/Stable already?.
Often people use apt pinning when they have mixed repositories in their sources.list.
For example a mixed Testing/Sid system.
The following guide shows the method i use for running a mixed system:

http://forums.debian.net/viewtopic.p...ing+sid+system

It might be worth posting your sources.list as well,so we know what exactly you are trying to pin.

Because i use even "external" repository, my idea will be to install program from other repository only when they aren't present on the main repository.

$ cat /etc/apt/sources.list

Looking at your sources.list i can sort of see where you are coming from.
If the package you require is not in the main repository,apt will install from an added repository such as Debian Multimedia automatically.
No need to use pinning for that.

Yes but i haven't checked if on the other repository there are some identical package but more update.
What i would like have, if avoid the installation package recent if exist version old on the main target, so i can avoid stability problem on the system because remain untouched.

If you want to get your packages from backports upgraded automatically the following entry in /etc/apt/preferences should be sufficient:

Package: *
Pin: release a=lenny-backports
Pin-Priority: 200


All backports are deactivated by default (i.e. the packages are pinned to 1 by using NotAutomatic: yes in the Release files, just as in experimental).

As for the other repos, I know debian-multimedia.org doesn't require pinning, the package versions are higher so aptitude/apt-get automatically installs from there rather than from lenny.

man apt_preferences
Ok, I lower futher the priority on the last repository.

In summary using apt-pinning isn't possible doing what i would like, patience.