Debian: Is it possible/safe to install packages from an older version of the repository?

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 did this once -- it was the image viewer gliv, which I wanted because it can do slideshows, but the latest version (in Debian sid) had a bug and terminated on startup. Once "downgraded", it worked.

In /etc/apt/sources.list, say it looks like this:

deb http://ftp.de.debian.org/debian sid main contrib non-free
# deb http://ftp.de.debian.org/debian wheezy main contrib non-free
# deb http://ftp.de.debian.org/debian squeeze main contrib non-free

In that case, I'd uncomment the last two lines, then run sudo aptitude update, then aptitude versions PACKAGE. Spot the version I like, and run sudo aptitude install PACKAGE=VERSION (type the version precisely as in the second column of aptitude's output).

When done, I might want to revert /etc/apt/sources.list so I'm just roaming one Debian release at a time.