Learn Italian: Il collega con il quale discuto il progetto è intelligente. - The colleague with whom I discuss the project is intelligent.

Yes! Con cui and con il quale both mean "with whom" and are completely interchangeable here. Using il quale (and its variations) is simply a bit more formal and has the added feature of explicitly showing the gender and number of the person you are referring to.

How do I know whether to use il quale, la quale, i quali, or le quali here?

The relative pronoun must match the gender and number of the noun it refers back to. Here, it refers to il collega (the colleague). Since il collega is masculine and singular in this sentence, you must use il quale.

Wait, why is it il collega? Doesn't the -a ending make it a feminine word?

Collega is one of those tricky nouns that can be either masculine or feminine depending on the person it refers to, but the spelling stays the same in the singular. We know this is a male colleague because it uses the masculine article il. A female colleague would be la collega, which would then change the relative pronoun to con la quale.

I learned that "to discuss" is discutere di. Why is it just discuto il progetto without di here?