Quote Originally Posted by Mariux Maxentius
I saw in the dedicated RTW forum of this site that, in order for the Marius reforms to come about, you need at least one "Huge City." I believe that's approximately 24000 residents. The last barracks available to build (check the cool technology tree thing in-game) allows you to build urban cohorts. Additional upgrades will open up more to you.

The senate gives you "the best units available that you can recruit," and later in the game those Cohorts are available for you to build regardless of your status with the senate.
To be precise, you need to build the Imperial Palace. Once it's constructed the Marius reforms will kick in. Oh, and if one of the other Roman factions builds an Imperial Palace first it will also start the Marius reforms for your own faction (and vice versa).