If your version of
libpango1.0-0 is causing problems, try to switch back to the correct one for squeeze, if possible.
apt-cache policy libpango1.0-0
50 http://debian.csail.mit.edu/debian/ testing/main i386 Packages
50 http://debian.csail.mit.edu/debian/ unstable/main i386 Packages
*** 1.28.3-1+squeeze2 0
500 http://debian.csail.mit.edu/debian/ squeeze/main i386 Packages
500 http://security.debian.org/ squeeze/updates/main i386 Packages
What version does
apt-cache policy show you for
apt-get install libpango1.0-0/stable
If apt complains about dependencies, try adding them to the command line, as packagename/stable. I don't know if this will work, but it is a reasonable first step.
To backport gimp 2.8 may not be so easy. I suggest you post that as a separate question.