A few extra pieces of information regarding tips given by RS. If you will end up around as maniac as myself and try to finish all of the campaigns in this game, keep in mind that in short campaigns you must be very speedy. You need not only complete the stated objectives, but also complete them before any other faction completes its own set of objectives first.

Unless you mod the game you need to win a campaign (short or long) with a roman family in order to unlock the other factions. I'd say that the order in which the roman families are listed is tied to early game difficulty if you follow the path that is most obvious for the given faction. The Julii have the easies early-game opponents but the natural route for expansion doesn't lead to very rich territories for a while, so you may struggle with the economy. The Brutii face some tougher enemies but can gain relatively quickly some of the richest areas in the game. The Scipii have a more complicated start in front of them which also requires a good deal of naval planning, but they can reach some very wealthy provinces if managed right.