In fact superfactions are more complicated than expected.

In principle you can assign this role to any faction, and (as the senate) it will be no expansionist at all.
It is not necessary that one superfaction is over three other factions. Even it is possible that one faction is superfaction of itself.

Missions are hardcoded to roman factions, and hence if a non-roman faction is included in the superfaction system, this faction won't receive missions.

I must say that using superfaction system with non-roman factions makes the game rather unstable, and CTDs can occur quite often. Because of that we reduced this system to the minimum to modulate the agressive character of some factions.