RalphLambrecht
Subscriber
...Is it a problem that I'm doing this on a neg that has already been selenium toned?...
Indeed. Selenium toner converts silver into more stable silver-selenide, sulfide toner into silver-sulfide. In case of an already selenium-toned negative, sulfide can only convert residual silver into silver-sulfide.
You need to do this with an untoned negative, but again, make a print before and after to appreciate the effect.
By the way, the rotten-egg smell is coming from the hydrogen sulfide gas produced by sulfide toners. It is toxic in higher concentrations. Do this only in a well ventilated area. It can also fog photographic materials. There is no need to do this in the confines of a darkroom, because it can be done in daylight.