sid is the unstable branch and therefore comes with the associated risk of running software that could get broken (usually not IMO, but YMMV)
It is possible to run a mixed apt setup using a preferences file, that way if you just need to install a piece of unstable software you can append your apt-get line, for example :
apt-get install -f k3b/unstable
I good link to get you started with this process is on the Libranet, a commercial version of debian, support forum.
http://forum.libranet.com/viewtopic.php?t=1420
The setup explained there is only libranet specific for those apt sources under the Libranet heading and can be omitted easily if you are not using that distro.