There is an easier way... Go to C: Program Files/ The Installation folder for example (The Creative Assembly or Activision)/ Rome Total War / Data / World / Maps / Campaign /
imperial campaign / descr_strat.txt It is the second one from above on the left side. Open it and you will see "Playable, Unlockable, and nonplayable." Copy the nonplayable factions into the playable faction's column. Don't do the senate or the slave, because it will freeze with the senate when you go to the senate floor. With the rebals I am not sure, but I have heard that they also freeze the game. I hope this helps.
FoR
Its hard for me to remember my age, since it changes every year.