To answer your first question I have to ask one myself: did you get the "Chance to Power" message yet? If so, you can attack the other Roman factions any time you like. However, if you choose to start the civil war by yourself, either by directly attacking the Senate or another Roman faction, an all-out civil war breaks out, and it´s each for himself. If, however, you choose not to attack the Senate, I´m told (I´ve never experienced it myself, sadly, because tha´s how it is actually supposed to work) you´ll sooner or later get a message from the Senate, demanding your faction leader to commit suicide. You can obey, which will buy you a couple of turns before the message is repeated, over and over again in time, until all your family members are dead, or you can choose not to obey and be outlawed.

As for your second question, actually that´s a misconception down to bad presentation of data. You see, army and family member upkeep cost are distributed among all your settlements according to settlement size, the bigger the settlement the higher the part it has to contribute to your war chest. That also means, small settlements only have to contribute very little, which makes them seem more profitable - they´re not! Your big settlements have high tax incomes (the overall tax income, among other things, depends on the numbers of taxpayers, after all), let them rebel and kill all the payers off and you´ll lose a mayor income source. It will look like the settlement became profitable, but the screen you´ve got to pay attention to is the financial overview on your faction screen. Forget the figures you see on the campaign map, they´re screwed.