Learn Spanish (Latin America): ¿El jefe es estricto o impulsivo?

No! The word o (or) only changes to u when the following word starts with an o or ho sound (for example, siete u ocho). Because impulsivo starts with an i sound, the o stays exactly as it is. Be careful not to confuse this with the rule for y (and), which is the one that changes to e before words starting with i or hi.

Why do we use es instead of está?

We use es (from the verb ser) because being strict or impulsive describes a person's core personality and character traits. You would use estar for temporary states, moods, or conditions, like if the boss were currently angry or tired.

If the boss is a woman, how does this sentence change?

You need to change the noun and make both adjectives feminine to match. The word for female boss is la jefa. The sentence becomes: ¿La jefa es estricta o impulsiva?

When speaking, is there a pause between o and impulsivo?

Usually, no. In natural spoken Spanish, vowels across word boundaries tend to link together. The o flows directly into the i of impulsivo without a harsh break, sounding almost like a single connected word.