the nice station is fine but the monaco train station can be a little difficult if you're not familiar with it, it's built inside a mountain and exiting can be a little confusing with lots of stairs/escalators to get down to the port -- to get to where the large cruise ships dock you'd want to take the stairs in the middle of the platform (these stairs are located basically right where the back end of the train stops), take the two long escalators down, walk left through the corridor to get outside, then walk around to the other side of the port (~15min walking once you're outside)

in the uber app you can request a van if you need more space (for more $ of course)