LinuxMCE will work with many external security systems - some work out of the box, some may need work on your part.
Also realize that LMCE has its own built-in security system that you can piece together yourself. Just add sensors from ZWave, Insteon, X10, etc. and LMCE will manage them under its own security system seamlessly. You can then even add cameras that tie into the security system and motion detectors - again all managed under LMCE.
Bottom line is that you have a lot of choices. Take some time to think about what exactly you want your security system to do, the kind of components you want (window sensors? door sensors? motion sensors? cameras?), and the level of redundancy you want - then post back here and we can help you narrow it down.