Debian – Why is mono-complete completely outdated on all versions of Debian

In this case, yes, it's possible and safe.

As debian keep dependences tree for each requested package.

At all there is still a risk that some libraries could not exist in two different version together in same installation, due to conflict (port reservation, device driver and so). In this kind of situation, apt would prevent you and ask for what to do. (Come back with another UL question in this case;-)

You could add squeeze.list to source.list.d

(Care! New versions of APT will ignore filename not ending by ".list".):

cat <<eof >/etc/apt/sources.list.d/squeeze.list
deb http://ftp.be.debian.org/debian/ squeeze-updates main contrib
deb-src http://security.debian.org/ squeeze/updates main contrib
eof

add a default directive to /etc/apt/apt.conf.d/

cat <<eof >/etc/apt/apt.conf.d/99squeeze
APT::Default-Release "wheezy";

Than use -t switch to apt-get for overriding default config:

apt-get -t squeeze install scim-pinyin

I don't understand why you are doing here. Why do you have a preferences setting for stable at all if you are running a stable system? As far I know, no preferences setting is necessary for stable in that case.

You don't explicitly say whether you are running a stable system (you really should say so), but if you are not, then I really have no idea what you are doing.

And if the release is on stable, then the usual thing to do for testing and unstable is to set their preferences to less than 100. I usually use 50.

And if you want to downgrade to stable, just do the following (assuming sane settings like the ones above) to downgrade pkgname1 and pkgname2:

apt-get install pkgname1/stable pkgname2/stable

This sets the specified packages to the target release stable.

Incidentally, mixing testing and/or unstable packages with an unstable system is generally a bad idea unless you know what you are doing. Some of the time it is Ok, but most of the time you need to use backports, either from Debian, or self-made.