I don´t think he can do that, for the same reason as he´s unable to attack his fellow Romans. I believe the Popularity with the people (not the Senate!) is too low. As far as I know, you can only attack the other Roman factions after you got the "Chance for Power" message, telling you that you´ve got enough support from the plebs to become Emperor.

But just how did you manage to get the Senate to outlaw the Bruti and Scipii while staying on his good side? I´ve never managed to do that, perhaps because I expand too fast (?). How many provinces did you have when the others were outlawed? And how many had they?