If you are thinking of the Persians in Alexander Total War, you might find there are not enough players as few people seem to have bought that expansion. Parthia in RTW or the Sassanids in BI (both Persians of a kind, AFAIK) might be more feasible and arguably interesting.

In terms of running a PBM here, the key thing is getting half a dozen reliable players signed up before you start and then taking an active role keeping the game progressing. The most common problem with PBMs is lack of interest, often when someone takes the savegame and then disappears for weeks. Introducing some houserules, as Tiberius says, would be good. The one's I'd recommend in particular are no retraining, no extermination and historical armies (the RTR forums have a great thread in their tips and tactics sub-forum on that theme). I'd recommend players reign for only 20 turns - anymore and it is too exhausting (and exclusive).

You might think about going with a non-Roman faction in the Metropolis/Naval mod for RTR Platinum, as about 9 of us have that currently installed for the Will of the Senate campaign and it seems to be favorably received compared to the vanilla game (see the FAQ for that campaign for installation details).