As a general rule it doesn't make much sense mixing Debian-stable and Debian-testing
(if it works or not is not the question. The meaning of stable means it *is* stable. That's its purpose. If you don't care about that you can run testing).
Both, testing and unstable are quite stable for a while now. That said: the bigger your environment is, the more vulnerable it is. You say you want kde, so in that case it might be different.
Else i don't see much reasons not to run Debian-testing or Debian-unstable or a mixed testing/unstable system. At forums.debian.org is a how-to do that.
You can't install unstable. You got to install testing and upgrade to unstable.
If you want to test Debian-live test Debian-live (and not Sidux, crunchbang or whatever):
Sidux?...sometimes it's better not to speak. But it is a good OS.