It seems to me the best way to expand is by conquering neutral provinces. Its possible to over expand but not if you keep decent armies every so there in your empire, to flat out rebellion and dissidence. The worst way to expand is to get involved in a war you cant win, most nations in this game are hard nuts to crack. And keep up your cultural (purple color) buildings, that helps keep populations loyal and in check.
Take it from my faliled carthage first game, expansion can be fun but beware oh the different cultured people (they would rather rule themselves )