"Chance for Power" and "Marius Reforms" are two totally different thing. The chance for power indicates that the people of Rome overwhelmingly support you and you can begin the Roman Civil War and destroy the Senate.
The Marius Reforms are a series of military reforms implemented by Gaius Marius that changed the Roman armies from an army of citizen-soldiers to a professional fighting force.
For instance, before the Marius Reforms, your main unit was likely Hastati. However, when the Reforms come into effect, you'll be using Early Legionary Cohorts, Legionary Cohorts, etc.
In simple terms.
Chance for Power = Attack Rome and kick the other Romans around a bit.
Marius Reforms = New army
And yes, every Roman faction gets the Reforms, as I learned as Germania when the Julii and Brutii both sent giant armies of legionaries at me.
Rome: Been on steroids since 279 BC!
(Seriously, how can any faction have so many troops without going bankrupt? Two full armies per city ain't cool, especially post-Marius!)
Macedon AAR