Milk instead of water for sourdough?
Adding milk works because milk is mostly water. It might even work better, as it also contains sugar (lactose) which will be eaten by your starter's yeasts and bacteria.

But it also contains other stuff, such as fats. And fats, after some time, get rancid. You probably won't want that taste in bread or pancakes. I would stick to water (which quite probably you'll find cheaper than milk).

Don't worry about rotten milk: your starter is already rotten. And it's a hostile environment for most microbes. It's very very improbable new microbes would survive there. But if they do, they'll be quite similar to the already existing ones.