Yogurt=lactose. Birds are not milk drinkers, they do not have the enzymes needed to digest lactose from what I have been told. I would stick to probios, but unless she has a poopy butt I wouldn't worry too much about it. I wouldn't put fungus cream on, I would keep her clean and dry and watch her for any other signs of a change in health (weight loss, poor feathering, listlessness, etc). If it's not related to lice or mites or skin irritation then it is something I haven't dealt with before. It is not related to the tape or from the loss of a few feathers, I am fairly certain. I trim and pluck silkies bottoms all the time to facilitate breeding and aid in fertility. In fact, all adult silkies are currently trimmed or plucked (depending on when they might show next).
I will hope Gaga is a pullet, but if she's a boy she'll be a nice black showgirl cockerel for you! I know you are working towards more showgirls, and either way he/she will be a good addition. I will check out the other thread for more recent pictures.
BTW the tape was not duct tape. I had tried to tape some lavender eggs to the inside of the box as a bonus. Needless to say it didn't go as planned. Lesson learned.