You have been mistaken by wikipedia which says that Marius the younger's wife was a Mucia. But this is wrong and has long been proven wrong by the best scholars (first of all Munzer).
I also think you are confusing 2 different Mucii Scaevolae from 2 different lines of the same family : the augur (159-88, consul 117 and teacher and mentor of Cicero) and the pontifex (consul 95 and murdered in 82 by extremist marians).
Cicero, in 2 of his letters (Brut 211 and ad Att 12.49.2) speaks of these people whom he knew very well since he was from Arpinum like Marius and was something close to a friend with Marius the younger and since he was also an admirer of the orators Crassus (consul 95 and censor 91) and Scaevola the augur (his teacher and mentor).
The Mucius who interceded for Marius against Sulla in 88 in the Senate and to whose house Marius the the younger fled and hid in 88 was the old Augur, not the Pontifex.
Marius the younger would not have had murdered and did not have the father of his wife murdered 6 years later in 88. His father Marius would also not have allied (by marrying his son) to a line of the Mucii who was allied with the Metelli, since the Metelli were Marius's enemies.
Mucius Scaevola the augur had only 2 daughters. One of them was married to an Acilius Glabrio and the other to Lucius Licinius Crassus, the orator admired by Cicero.
And Cicero explicitly wrote that the youngest Marius (whom Anthony had killed in 44) was the grand son of Crassus the orator.
Licinius Crassus the orator had 2 daughters too, both named Licinia : one married to a Cornelius Scipio (and mother of Quintus Caecilius Metellus Scipio) and the other married to Marius the younger.
Scaevola the augur was the grandfather of Marius the younger's wife.
As far as the great Marius is concerned, he did marry a noble patrician woman, Caesar's aunt. But don't mistake here too. At that time, this line of the Julii Caesares was not important. They were relegated, third rank nobles, though from ancient patrician family. In this mariage, it is not Marius who struck a good deal : it is the Julii Caesares who made the right bet on the new man who was just about to become the great hero of the republic. The mariage took place around 110 (since Marius the younger was born in 109) before Marius was elected consul for the first time.
But you are perfectly right about Mucia Tertia wife of Pompey : she was the daughter of the pontifex murdered in 82.
The same confusions are possible between 2 lines of the Julii Caesares :
- the one which was initially the most powerful and prestigious, with the consul of 157, then his grandsons Lucius (consul 90 who passed the law granting citizenship to the italians during the social war, and was Anthony's grandfather) and Gaius the orator, both murdered in 87, like their ally Marcus Antonius orator (Anthony's grandfather), because they had betrayed and turned against their former ally (and to a certain extent former patron) Marius.
- the one who was most closely allied with Marius and remained faithful to Marius, with the 2 brothers of the Julia married to Marius. These 2 brothers were Sextus Julius Caesar, consul 91 (thanks to the support of Marius) but was unfortunately killed in battle during the social war, and Gaius Julius Caesar the father of the future dictator Caesar. That is this alliance with Marius and the loyalty of the young Gaius Julius Caesar to the cause of the marians and of the populares which was one of the keys of his career, his popularity, and his successes.