There are plans in the works for "dogfight" type servers.
No ETA of course -but how quick that happens, I would venture a guess, depends on helping out NeoQB in other areas *cough-tech help-cough* to free up bodies to continue development of future content patch releases.
...and there is
*NO* 5v5 limitation/recommendation.
I suspect they are "popular" because it's the closest thing to a "dogfight-server" where missions take the least amount of time...but still have enough player-slots to be fun. That's why the bases are so close to one another.
<S!>
LOL...do not hold your breath on that dog-fight server(s). The company that made WWII Battle Tanks promised us MP servers also...now the game is dead and company...no support. Probably same thing to happen to this game.
[EDIT]: ...and =FB=Weeper has a Russian 6v6 server that was swamped most of the day. He also threw a 10 minute timer on the mission. That way, players entering do not have to wait long.