Using Kvatch Rebuilt with FCOM is as easy as enabling the two plugins that are for that. Well, one is for OOO, and the other specifically for FCOM (i.e., OOO and MMM, I imagine - usually that's what FCOM compatibility amounts to).
UL, it's practically done for you. From memory, there might be two or three compatibility plugins for OOO, so you'd want to use those (assuming you're installing all of UL) of course. MMM only requires compatibility plugins if you are using particular non-standard options from that mod.
FCOM... yeah, as anithinks said, basically. I suggest trying OOO plus a few other mods first. Once you're fine with that, perhaps FCOM will seem a bit less intimidating. Also, get to know Wrye Bash before you even look at FCOM. If you're comfortable with WB, and can read English, I can't see any major issues arising. Provided you follow the instructions as written, that is.

edit: The current release thread for FCOM is always the first place you should look, for FCOM installation info. Possibly the only place. Depends if you're installing with BAIN (I recommend this, but it's not the official way... yet

) If so, Tomlong's site and Psymon's guide would be key places to look for further guidelines and advice.