1 Can I use PanoCalc for fisheye lenses ?
2 What is the best kind of string to use ?
3 How much overlap should I use ?
Both the Field of View and the panorama calculations are based on normal rectilinear lenses. Fisheye lenses are much more complex and really need treating on a case by case basis. The 8mm fisheye is covered in detail in detail here. The Nikkor 10.5 fisheye is another excellent lens for panoramas.
In principle ordinary cotton thread is best as its almost invisible and quite strong enough. However its hard to handle and untangle and for that reason something a bit thicker is preferable.
The answer depends on what is in your pictures. You need overlap for several reasons. (i) To provide enough overlap for the blending software to to its magic. So the sky will tend to need more overlap than houses to avoid visible banding..
(ii) You need overlap to provide identifiable stitching points. So if you have 2 pictures with only sky in them no amount of overlap will provide stitch points. (iii) Another factor is the number of people etc around you and their distances and whether you want to include them or not.
But if you wanted a simpler answer I suggest you try between 25% and 30% and and see how it goes !