i'm using a tascam 1/4"TRS patchbay. I was thinking of also patching my composite video signals through this patchbay. Does this affect video quality? Are audio and video cables interchangeable?

Composite video cables with RCA connectors are indeed interchangeable from audio to video. I prefer not to skimp on their quality and try to use fairly thick, stout cables with gold connectors, as short as possible.

I like to use the pre-made "snakes" that have three cables bound together, with color-coded ends (yellow for video, red and white for stereo audio).

Gold plated contacts are important because gold is the only material that garantees good/permanent contact in (almost) currentless connectors. Audio cables don't reduce video quality (bandwidth) if under 10 feet length. Very thin 10" versions introduce some general attenuation on video (75 Ohm impedance use)

Depending on what all you route through the patchbay you might have termination problems with video signals. There is also a chance (depending on cables and length) you would introduce a phase shift (colors are off). If it's just plain VHS you may be OK. But higher end analog formats might present problems that your clients would notice.

Jeff