Is it permissible to adopt the fatwa of a specific madhhab on one issue, and to adopt the fatwa of another madhhab on another issue, such as adopting the Hanafi madhhab's view that swallowing phlegm does not invalidate the fast, and the view of others that water entering through the two private parts does not invalidate the fast?

We have not found any statement from the Malikis or others that obligates drying the anus and genitals after washing. However, the Hanafis explicitly stated that excessive purification (istinja') to the point where water reaches the place of enema invalidates the fast, because at that point it is considered like an anal enema which breaks the fast.