Pharmaceutical hormones can be produced from any sterol source. Sources other than soy, such as wild yam, are also used. But these sterols have to go through a number of processes to convert them to actual hormones, and these do not occur in the body.
No, soy yogurt is not a problem for breast cancer. A number of studies have shown the anti-cancer effects of soy phytoestrogens, and many of the herbs and foods used to treat cancer are also phytoestrogen rich. Phytoestrogens do not promote cancer, they inhibit it by blocking estrogen receptors so that real estrogens cannot affect the body. They are not 100% blocked, but antagonization of any estrogen in the body is a help.
As far as Candida there is not a problem as long as it has been fermented long enough to destroy the sugars present. the bacteria and acids formed will actually help suppress Candida growth.