Some questions.

[Brandy Blue]

You can get the legionaries auxillaries etc. after the Marian reform. That happens when at least one Roman controlled city has the most advanced possible government building. I think you need a pop of 24,000 for that to be possible. Probably you will be able to do this before the other Roman factions, but that is not such a big advantage as you might think. This is because as soon as one Roman faction gets the Marian reforms, the others get it free.

As for attacking other Roman factions, you cannot do it until either your popularity with the people is high enough for you to get the "bid for power" event, or the Senate becomes so afraid of you that they order your faction leader to commit suicide. Just refuse and they outlaw you. To become that popular with the people (or unpopular with the senate) just keep capturing more provinces. I think you need about 20-30 but I'm not sure.

As for becoming Emperor, well I never actually completed a campaign but I think you need to accomplish 2 goals:

1: control Rome
2: control 49 other cities of your choice