true, a contiburnia would be more of a squad.
another thing about a Vexalation though is that it was usually a detachment of the main army sent elsewhere. So like if the main Praetorian unit was stationed in...oh..Nordskogen lets say and a detachment was sent west to scout, or guard or do whatever, then it would be a vexalation. They carried there own Vexaltio banner that sigified they were part of the greater Eagle but they were seperated pro temporum.
A Cohort would indeed be a large entity but sometimes groups have been known to take names like that in the SCA because face it, were not even going to be a true century strength but we like to reenact as much of this as we can, That's how we can justify having a Centurian or even an Optio when we only field about 20 fighters. We reenact and basically pretend we are in the days of Empire so why not fudge a little on the numbers as long as we still understand and recreate things as best we can.
So why merge? First off we are Romans. Its completely period to have different squads, cohorts, legios etc spread out and then coming together. By being one big group we also develop a bigger and better (and more glorious) identity. We share resources more, we camp together when we can and have a bigger, grander camp, we have more unity. We plan and participate together in our raids and surprise attacks on the barbarians (other groups..). I base a lot of this on what happened with my old household. While Japanese it was VERY much like what we are doing now in that it is personae specific and when we grew into having Shiro (chapters) all over the U.S. I remember traveling to other Kingdoms and because of the household's reputation I was even able to sit at High Table once and had key positions in wars.