For Sigma 8mm full circle fisheye lens
go ahead and stitch the 4 images together. Then convert the equirectangular stitched image to 6 cube faces. Take the 5th cube face tile [I think] into photoshop and clone or patch tool out the tripod head base. The re-assemble the cube faces back to an equirectangular 2:1 image.
Yes, you can take the camera off the tripod and take 1 nadir down shot. On the panorama head 4th click stop remove the camera from the pan head and roughly hold the camera at the same height as it would have been while still on the panorama head.
There are some other techniques. Maybe someone else would know more about those.
Forum Moderator for
EasyPano - Panoweaver