I dive a 3-person buddy team quite often. It takes a great deal of vigilance/situational awareness, and communication...especially in conditions with very low vis, e.g., 5 - 10 ft. In really poor vis, we have to dive shoulder-to-shoulder with the middle person being a central point of visual contact. We usually try not to dive under such conditions, but sometimes vis can deteriorate dramatically in a short period of time.
With respect to formation, we tend to switch off duties from dive to dive, but it is helpful to designate a fearless leader.
If I were in your shoes, I would keep the 3-person buddy team of known dive buddies rather than take on an insta-buddy on a boat. Otherwise you're gambling with your vacation! Consider how much pre-dive prep you'd have to do with a complete stranger and how no one will probably be familiar with the site. It's a recipe for "issues." I like doing worry-free dives with people whom I know. Their actions are predictable, communication is clear, and we all look out for each other. That just isn't the case with strangers. (Don't get me wrong, though. I enjoy diving with strangers and/or novice divers at various local dive sites...when my vacation isn't on the line.)
Have a great vacation!
