There isn't much information about Marius in the ancient sources, not the type that you want. He didn't write his own memoirs, none of his followers seemed to either. The majority of information about him seems to have come from the accounts of Catalus, Rutilius Rufus, and Sulla's memoirs, all of which would have been critical of him, and all of which were lost in history. I'd be wary of using Colleen McCullough books as a source; though they are very entertaining she used a lot of creative license in describing his life. I recommend reading
The Crisis in Rome to get the info related from the sources (very accurate book) about his life and then using your own imagination to fill in the blanks in your novel.
Some key parts of his life:
- He served as a cavalry officer as a youth in Numantia and was recognized for great bravery by fighting in single combat and winning. So he was rich growing up (a member of the Ordo Equestris), not poor as Plutarch describes. But there are no indications that he was as rich as Colleen McCullough made him out to be. The hypothesis that he owned silver mines in Spain was due to another Marius, a century later, who owned mines. But Marius was hardly a rare gens name among the Latins and Italians.
- His excellence was recognized by Scipio Aemilianus, who made him one of his contubernales (tent companions) during Numantia, which along with the praise he received during that war near guaranteed him a public career.
- He never served in the east (Macedonia, Greece, Asia), nor in Africa. Most likely the majority of his service was in the Spanish provinces, fighting the various small scale tribal uprisings and near nonstop banditry, basically Rome's long term Vietnam War. It was probably this experience that made him valued as a counter guerrilla expert and why he was brought in on the staff of Q. Caecilius Metellus in the Jugurthine War (in addition to his experiences under Scipio Aemilianus, who was known as a drill master and very organized commander).
- He probably also served in Transalpina Gaul during the 120 BC decade during the various wars to subjugate that area, as a military tribune and possibly a legate.
- He never triumphed before 104 BC (over Jugurtha). Marius had success as a Praetor fighting bandits in Spain (increasing his abilities fighting unconventionally) but he did not receive much in the way of spoils, nor a triumph in Rome (nor Ovation).
- His family were the ancestral clients of the Caecilii Metelli, though Marius would show great disloyalty to them as a tribune and as a legate. The precedent for a client betraying his patron on achieving senatorial rank was made by Marius, according to Cicero.
- Marius' skills as a commander was his ability to operate unorthodoxly, relying largely on sound stratagems (high ground, flanking maneuvers, and of course deception). He was known as a barbarous commander who showed no mercy to the enemy (man, woman, or child), who sometimes operated outside the moral norms of the day. He was often overly cautious in battle, and rarely fought a battle unless it was on his terms. I wouldn't describe him as overly audacious (like Alexander or Caesar). He was an excellent planner.
- He was absolutely immoral when it came to politics, starting at his earliest political career. A firebrand demagogue from the start, he was probably also a closet supporter of Gaius Gracchus. But it should be noted that as much as a popularist as Marius was, during his seven consulships, he did little to actually help the plight of the average Roman. Most of his great reforms only benefited his personal soldiers, many of whom he had turned into clients.
- He had a younger brother, Marcus Marius, that would have probably served with him during many of his campaigns, and in Africa, and then went on to be a successful Praetor and army commander in his own right fighting in Spain. It was this brother's adopted son Marcus Marius Gratidianus who would become a player during the bloodshed of the early Marian-Sullan civil wars.
- Marius was a strict disciplinarian but was also fair. When necessary, he would push to have executed subordinates suspected of treason or crimes, but he also would reward some who performed questionable deeds if he thought the reason good enough (Trial of Trebonius).
- Marius had zero respect for the Mos Maiorum and violated it at every turn whenever it benefited him.
I've studied Marius extensively, so if you have questions, feel free to hit me up.