That's the timing reason why small groups shouldn't leave first. Now lets talk about separation from other jumpers. First of all, anyone who counts on vertical separation for safety is out of touch with reality. I see people in freefall at 1,500 feet and lower routinely, so just because someone plans to open at 2,500 doesn't mean you should bet your life on it. Everyone needs to open in their own column of air.
Horizontal separation is the only guarantee of security.
I repeat, horizontal separation is the only guarantee of safety. Vertical separation is a nice idea but cannot be counted on since a minor loss of awareness or a long snivel will eliminate it instantly, not to mention a premature deployment. Now, a quick digression about fall rates. Follow these categories out or time their videos if you don't believe me. Light freestylists doing routine freestyle do not fall significantly faster than a fast falling four way.