Sensei 30-37mm Aluminum Step-Up Ring

If your filter/attachment is larger than your lens, it's a step-up. In this case, your fisheye attachment is 37mm and your lens' front diameter is 30mm, hence, you'd need a step-up ring.

This just steps up the filter thread on the front of a lens from 30mm to 37mm for adding accessories to a lens such as filters, lens hoods etc. It has no relation to the focal length of a lens. It is not clear what you mean, please submit a more detailed question about what you are trying to achieve.

If your filter/attachment is larger than your lens, it's a step-up. In this case, your fisheye attachment is 37mm and your lens' front diameter is 30mm, hence, you'd need a step-up ring.

Yes, you can stack 2 filters with the same size thread but beware of vignetting (darkening of the edges) with wider angle lenses.

They are about 5mm thick which is about the same as a standard UV filter ring.

Yes, the black color and finish covers the entire step up ring.

The Sensei 67-95mm Aluminum Step-Up Ring has course 1.0 pitch threading on the 95mm side, and standard 0.75 pitch on the 67mm side. This is fairly standard for these sizes.

Yes, it's meant for any circular filters with 82mm diameter.