In one vs one situations, assassins are more dangerous than general, but when it comes to 3vs3 or more, having a general on your side that provides an interesting aura (speed, regen, ....) is a great asset.
the matchmaking system chooses the team-size mainly on the numbers of players available when you start the match, and then chooses a map accordingly. So there is no specific team size in the pantheon: you launch a pantheon match, the system search allies/ opponents and choose a map that can host that number of player.
The system also chooses the game mode.