Soft focus for 35mm portraits ? First, control your lighting, exposure and development.
Then, keep an embroidery hoop with a piece of black silk (not nylon) stocking in your desk drawer.
Use it under the lens for a percentage of the exposure.
Black silk stockings are the ideal diffusion media, and you can use them over the taking lens as well.
Not to mention fun to obtain.
The catch is with 35mm that enlarging a diffused image changes the whole bizniss of diffusion. You can accomplish most of the job with generous exposure, gentle exposure, and simple lighting.