can one use regular speaker wire to hook up a sub?

is there a difference between a sub cable, composite video, and analog rca stereo cable?
they all have rca ends so if one really wanted to could they be interchangable?

A powered subwoofer cannot be connected with standard speaker wire (at least not if you are using the subwoofer output of your receiver or processor).

A subwoofer cable is an analog audio cable, so there is no difference between the two. A composite video cable, coaxial digital cable, or component video cable will all be the same: they use a 75ohm connector, something that is not essential for an analog audio cable.

what happens if you use a subwoofer cable to connect into a composite video input for video use or visa versa?

75ohm cables can be used for analog audio purposes without any harm. A non-75ohm cable used for video or digital audio is not recommended.

The problem that you might run into using a fairly long regular audio interconnect is that you will have an impedance mismatch at each end of the cable where it connects to the pieces of gear. At low frequencies, generally taken to be those at which the cable length is less than 1/10 the wavelength of the highest frequency of interest, impedance mismatches can be safely ignored. However, at higher frequencies if there's an impedance mismatch some of the signal is passed and some bounces back. With a mismatch at both ends we'll have a bounce from each end.

Lets do a little math here.
Assumptions:
1080I resolution, 30 foot cable.

30 frames per second of 1920x1080 resolution is just over 62,000,000 pixels per second. Since it's analog, each pixel is represented by a single signal level. The speed of the signal in coax is about 1/3 C, or 10**8 meters/sec. This puts our wavelength at about 1.6 meters. Roundtrip, our cable is about 12 wavelengths. This means that our first ghost image will appear about 12 pixels to the right of the intended image. Naturally, this will be most visible with a sharp vertical edge next to a uniform color field.

The moral of the story:
Like Gonk said, 75 ohm for analog audio is fine. Analog audio for digital audio or composite/component video will probably work but not particularly well.

thanks for explaining