I have talked about the Marian reforms before. A follow-up down the page talks more about the reforms themselves.

In short: It is more reasonable to see the Marian reforms as the culmination of a long process of transformation and professionalisation of the army. The man himself still played an important role in finalising these changes, and introducing other, smaller innovations, but it indeed was not an instantaneous change, and others also played a role in these changes. (i.e. the individual weapons drill introduced by Publius Rutilius Rufus.)

As for the second part of your question: Well, yes.

Whatever had changed about the Roman army in Marius' day, the one thing that remained exactly the same was the higher echelons of leadership. The same men from the same families gained command in the same ways.

These men had greater leeway to pursue their own wars, now that they were commanding longer-standing armies far away from Rome and more loosely under the senate and people's control. If anything, military achievements became more important to reach the highest ranks. Witness Cicero, pursuing his little border war in Cilicia. (mostly thanks to his brother Quintus, but eh.) Witness Crassus going off to get himself killed in Parthia, because he fears getting eclipsed by the glories of Caesar and Pompey.

All these men served in the army in their youths. Cicero served under Pompeius Strabo (father of the more famous Pompeius Magnus) during the Social War. Said famous Pompey likewise served under his father, and then drummed up a army under his own authority to support Sulla in the civil war. (Pompey never was one for convention.) Crassus likewise achieved some prominence by supporting Sulla in that civil war (and then by getting rich by buying the estates of Sulla's proscribed and murdered political opponents.) and later, famously, beat down Spartacus. Caesar won the Corona Civica, one of the very highest awards for bravery, for his deeds during the siege of Mytilene as a youth.

It is only in Imperial times that the traditional cursus honorum starts to take on a different aspect.