Pax, the rating from worst to best for interconnecting audio/video is like so:
1) Coax (actually, coxial cable is ANY shielded cable with a single inner conductor, but here we're referring to RG-59 & RG-6 w/F-connectors, that carry an RF signal, with both video and audio signals)
2) RCA interconnects, with the video on a single shielded cable, and the audio on a) a single cable carrying mono, b) two cables carrying stereo, which can be encoded with ProLogic-decodeable surround sound, c) a single cable or an optical (fiber-optic) "cable" carrying a digital signal, which can contain anywhere from one to six or seven channels.
3) A combination of an S-video (which itself comprises two shielded cables, with the black-&-white info (the Y-component of the video signal (luminance)) on one and the color info (the C-component (chrominance)) on the other) and an RCA cable, with the same possibilities of the audio as in #2.
4) Component video, of which there are a few varieties, which can use three, four, or five shielded cables (I won't go into the differences here), along with the same choices of audio as in #2.
Now, to answer your Q's:
1) Yes, the cables are the same, in as much as the quality itself being the same. The Monster cable is thicker because it's a Monster Cable cable. What makes one cable a video and another an audio is how the provider lables it. If I had one good cable and one poorer cable, I'd use the better one on the video.
A quality shielded cable can carry digital audio or video, although, in theory, a digital cable could be a piece of crap, because the nature of digital signals is such that, if the signal arrives intact, it contains all the info needed to re-construct the complete audio signal.
Video is (except for the content of DVD and digital recorders) is an analog signal, so the cable used is required to be of high quality. However, as a matter of course, it's a preference to use a higher-quality interconnect even with digital audio for physical reasons, too.
2) In my opinion, yes, and the longer the cable, the more so.
