I'd actually not recommend k-lite
K-lite = bad idea
without getting into too much detail, k-lite is infested with multiple versions of the same codec.
You set yourself up for trouble any time you have more than one codec (or more than one version of a codec) installed.
as for editing flv,mp4,etc...
(glorious flashing banner is also a link)
this applies for nearly every modern editing program (the sony vegas banner was easiest to find)
the same thing goes for h.264 files...not sure about .flv since I'm not familiar with how it goes about encoding