Alex, soft focus lenses work with spherical aberration where a sharp "core" image is overlaid by a out-of-focus one. The proportions of "sharp" and "soft" are exquisitely sensitive to f-stop. With my 450mm meniscus lens f9, f10, f11, and f12 all look different. Close study of the ground glass is needed to catch the optimum aperture for the effect desired. The learning curve requires persistence and a tolerance for failure.
Soft focus usually stands for "impressionism", "dream states", "luminosity", "nuance", "nostalgia", that sort of thing rather than a detailed literal rendition. Cynics may suggest that soft focus lenses were invented to generate flattering portraits of older women. I reckon that soft focus and wet plate don't go together. The signature of each process is too strong to abide the clash.
Here is a resource (sorry, can't devise a link) that can be found online and treats the subject exhaustively:
"The Soft-Focus Lens and Anglo-American Pictorialism" by W R Young. This is a PhD thesis submitted to St.Andrews university in 2008. Lots of reading!