it is possible to unlock all but the slave factions without playing the prologue.
I have tried the slave faction in the past but it leads to crashes and instability.
This work round has been used by many others since the game was originally released and I know of no problems (except see later)
Creative assembly have never expressed any problems with people modding the files as long as you take responsibility for doing so.
In other words any alteration to game files is always at the risk and responsibility of the owner/user of the game

go to the following folders (or use the search function in the rometotalwargoldedition folder);-
Metaboli
games
rometotalwargoldedition
data
world
maps
campaign
imperial_campaign

save a copy of
descr_strat
as
descr_strat_o
this is so you have a copy of the original file with factory settings so any name you want as long as it is different. If you have any problems then simply rename this file as descr_strat and overwrite the modified copy.

Open
descr_strat
copy and paste all the factions from unlockable and nonplayable sections
into the playable sections.
Except the slave faction
save the file as descr_strat

When you start the game then you will be able to play as any of the factions bar slave.
This means that you can start as SPQR in Rome.
Which is actually quite a challenge as the only logical thing to do is start the civil war straight away.

stability problems;
do not open the senate window if playing SPQR as this crashes the game
sometimes the game crashes or seizes when settlements are left without a garrison.
I know of no others but that does not mean there aren't any.
I know of none which cause any permanent changes or damage to the computer
Please do not contact metaboli about any issues if you do modify the game as they cannot help. your responsibility.

Marian Reforms
this allows you to build what most people think of as 'normal' Roman legionaries.
It happens when any of the original Roman held cities, except Rome itself, become huge.
So those held by the Julii, Brutii, Scipii factions on turn one.
It is possible, even on harder settings, to bring about the reforms by around 240/230BC. Just grab a city and pour resources into population growth, do not exterminate or occupy conquered cities but enslave. This redistributes the population and promotes growth.

the following have been used in the past by many others as well as myself and I have taken from several sources.

"correct" pre-Marian legion;-
4 velites
4 histatii
4 principe
2 triarii

general
2 equites

mercenaries as requires

"Correct" Marian reform legion;-
10 legionary cohorts consisting of

1 1st cohort (should always be placed on the right flank)
3 praetorian cohorts
4 legionary cohorts
2 early legionary cohorts

General
2 roman cavalry

7 assorted auxiliary troops and mercenaries to make up to 20.

Garrison of Rome;-
(which should never leave Italy)
up to 7 Urban Cohorts and 1 1st cohort

enjoy!