Learn Croatian: Želim popričati s tim novim kolegom. - I want to talk a little with that new colleague.

The prefix po- here adds an attenuative meaning, which indicates doing an action just a little bit or for a short time. While pričati means to talk or to speak (imperfective), popričati means to have a little chat (perfective). It makes the request sound casual and low-pressure.

Why do the adjectives tim novim look masculine, but the noun kolegom seems to follow a different pattern?

The noun kolega (colleague) is a classic trick word. It refers to a male person and takes masculine adjectives and pronouns, hence the masculine instrumental forms tim novim. However, because it ends in -a, it declines exactly like a feminine noun (such as žena). Therefore, its instrumental form is kolegom.

Could I use the verb razgovarati instead of popričati here?

You could, but it changes the tone. Razgovarati is imperfective and translates more to having a conversation or discussion. It sounds more formal or serious. Using the perfective popričati specifically communicates that you just want a quick, informal word with the person.

Why is the preposition s used here instead of sa?