Is pre-seminal fluid (madhy) during intercourse considered among the minor impurities (yaseer al-najasaat), and is it excused?

If you adopt the view of Sheikh al-Islam concerning the forgiveness of a small amount of impurity in all cases, then what is considered "small" is determined by common practice (urf). If you are in doubt about whether something is small, the general principle is that it is forgiven until you are certain that it is a large amount. Imam al-Nawawi states that a small amount is what people find acceptable, and if a worshiper doubts whether it is a small or large amount, the preponderant view is that it is treated as a small amount.