I can create a least cost path (LCP) using a shapefile of points over a "cost surface" raster - But can I do polygons and not points?
This is not the same as the LCP between the closest points of each polygon. You'd have to know the best path from between the polygons "as a whole". See the illustration.
I want to find the LCP for a conservation corridor between "cores". Let's say you have a to get from on area to another area, and there's a mountain in between - the best path is around the mountain, and from a certain exit and a certain entrance.