Apt offers pinning support. That will allow you to mix and match packages based weights you assign them. It's a useful feature. You can even 'weight' a group of packages in such a manner that it allows you to 'roll back' packages to earlier releases and such things.
But in terms of 'stable' using apt-pinning to pull in packages from Unstable or testing is not useful. The problem you run into is that the way Debian does dependencies is that when they upgrade some lower-level package they re-compile everything then have the new packages depend on the new lower-level package. This is probably not necessary as long as the developers of the low level packages are not huge dicks about breaking ABIs, but it does avoid the need for Debian to care when libraries don't bother to stay compatible with themselves.
The effect of this is that when you pull in packages from Unstable you will be forced to upgrade huge swaths of your OS.
If you want to mix and match packages the best approach I found is to use backports.debian.org and/or backwards port the packages yourself using deb-src files. Despite what the Gentoo folks may say Debian does make handling/building/install source-based packages fairly easy. Using that approaches I have always been successful.
Although I expect that forcing dpkg to install and ignoring dependency tracking will probably work fairly well in many cases.