There are some comments in the other forums that the 7125 is more musical than the 2200. This, and the advantage of simplicity, would drive me to pick the 7125 over multiple 2200s.

I haven't compared the two, so I can only go on other people's feedback and reviews. There are a number of great reviews of the 7125 out there, but very little on the 2200 at this point. Here's one example:

http://ultimateavmag.com/amplifiers/206outlaw/

I wouldn't sweat details like power conditioning or dedicated circuits yet. You've got much bigger issues in front of you right now, like auditioning LOTS of speakers and finding what sounds right to your ears. Choice of speakers makes one of the most significant impacts on the quality of your audio system.

Cables: You'll need one RCA for each channel of the amps you are using. If you have HDMI or DVI out of your cable box, use that for video, otherwise you'll want a set of three component cables for the video. I like optical connections for the audio.

Wait until you get the other gear, before you decide on cables for it--you want the best possible connection, but what is possible varies with the component.

With regards to stacking audio equipment, there are three considerations: heat, vibration, and weight. Heat is the first thing to plan around; you want good air circulation around your amp and don't want things to overheat in general. I'd recommend getting some kind of rack that allows for an inch or more of airspace around each component. A decent rack will address any vibration and weight issues, too.