5.4.2 does not include the fix for QTBUG-45753: http://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/quick/items/qquickscreen.cpp?h=v5.4.2 https://codereview.qt-project.org/#/c/111100/2/src/quick/items/qquickscreen.cpp This will most likely cause plasmashell to start crashing again, e.g. when you change display outputs.
I do not control EPEL and I can't force EPEL builds to downgrade certain libraries. IMHO you should raise this issue with EPEL folks, i.e. that EPEL builds should not switch RHEL releases until CentOS/Scientific Linux updates are released. I'll try to a response myself from IRC.
EPEL6 packages are compiled against the latest RHEL 6 release, which in the case for 1.18.4 is RHEL 6.6 (release 13-Oct-2014), see ftp://ftp.redhat.com/pub/redhat/linux/enterprise/6Workstation/en/os/SRPMS/glib2-2.28.8-4.el6.src.rpm. So yes, the EPEL6 package pidgin-sipe-1.18.4 will not work on CentOS/Scientific Linux 6.5.
Dear email@example.com: are you sure your RHEL6 installation is up-to-date? According to the build logs, e.g. https://kojipkgs.fedoraproject.org//packages/pidgin-sipe/1.18.4/1.el6/data/logs/i686/root.log RHEL6 packages are built against "glib2 2.28.8-4.el6" which has g_slist_free_full.