so...if I mount a 24X36 lens on a medium format camera...it will give me a wider FOV? 
is this how it works? I worked on a studio modifying (simple) camera equipment for special scenes but I do not know that this thing works.
All lenses have an image circle (or more precisely a cone) outside of which no light is transmitted, however that edge is not a sharp one. At small apertures the circle is at the maximum usable size. At large apertures, shadowing by parts of the lens body itself blocks some of the light and causes vignetting.
As it is a cone, if you have sufficient extension, the coverage on the sensor/film can be greater as magnification is higher, but this does not affect maximum angle of view, just crops what is being shown.
If you look near the edge of the image circle you notice that there is light fall-off and poorer imagine performance. Usually the lense is designed to have this occur outside the frame as much as possible, but often we hear that a lens is "soft in the corners". This means that not all of the available image circle is being used.
With a fisheye, if you can see the edge of the circle on one format, going to a larger format is not going to show you a greater angle of view. It can only show you parts of the image circle which previously landed outside the sensor. In most cases this is the worst performing part of the lens and has been excluded intentionally.
