ALL collected deb’s must then be manually examined with gdebi before installing them:
- new packages that don’t show unmet dependencies may be installed *ALMOST* safely (especially binary library files).
- updated (existing) packages that don’t show unmet dependencies *might be installed*, but may break some other packages, namely those depending on previous version of the package being replaced. My advice is, make a backup copy of the (original) deb file before proceeding, so you can rollback safely, and make a backup of your current version of any config files within the package (those in /etc/…. as well as those in your home dir)
- any package showing unmet dependencies must be examined. If dependency deb’s can be fetched, download them from the repo of the first package, and examine them first. Once all dependencies have been examined & installed, there is nothing preventing a package from being installed.

Please note though, that packages (and their maintainers) couldn’t and didn’t know about possible future changes in other packages. Thus, even packages that install successfully, may not work as expected. This is especially true if packages are particularly complex, depend on certain directory/paths/links/libraries/binaries to be present.

I wouldn’t even try to install (from another repo) anything involving: kernel, kernel modules, video drivers, networking, X system, etc, while I would say it is fairly safe to install applications and their libraries (beware of related GTK / QT library dependencies though).

But you’re not left alone! Whenever you can’t manage (or risk) to install a deb from outside your repo’s, you can always build them from source!

Backports *may* have something for you. Maybe.