Getting 90 cities with the polybian troops isn't that hard, but it is kind of a pain.
If you try to do it the honest way make sure your general is a Plebeius, otherwise you'll NEVER get the Popularis trait... I haven't been able to do it despite some people on the forum saying they have. the hardest part I think is getting him to be a Consul. I think you basically just have to be in over 20-30 battles and have your general survive.