I think what Gan proves is that having an actual Seven might be a step too far, and I hope we don't end up with a situation where Gan/Villa/Cally end up with Reed/Mayweather/Hoshi levels of nothing to do!
There are plenty of TV series that have casts larger than seven members and manage to develop them all well -- DS9, The West Wing, Firefly
, the first season of Heroes
, etc. It's certainly possible to do it. It's just that some shows choose to be more ensemble-oriented while others choose to keep a tighter focus on just a few characters.
And I feel ENT did balance its whole cast pretty well in the first season, but over time they tightened the focus to the core cast for some reason, perhaps at the network's insistence (since the network seems to have been responsible for most of the poor decisions behind VGR and ENT).