Learn Italian: Il conduttore parla troppo velocemente.

Usually neither! While it can mean a conductor in a scientific sense (like heat or electricity), in everyday Italian il conduttore is a TV or radio presenter or host. A train conductor is usually il controllore or il capotreno, and an orchestra conductor is il direttore d'orchestra.

Why does troppo end in "-o" here instead of changing to match a noun?

In this sentence, troppo is acting as an adverb modifying another adverb (velocemente). When troppo means "too" and modifies a verb, adjective, or adverb, it is invariable and always ends in "-o". It only changes its ending when it acts as an adjective modifying a noun (like troppi libri for "too many books").

How is the adverb velocemente formed?

In Italian, you form many adverbs by taking the feminine singular form of an adjective and adding -mente. Because the adjective veloce (fast) ends in "-e", its masculine and feminine forms are exactly the same. So, you just add -mente directly to veloce to get velocemente (fast or quickly).