cum... essent:
The consuls began their new term at the start of the new year in January.
CumCircum
Romae
A locative.
Loc
cum... mutati essent... accepissent:
There is an implied et between these two verbs.
Nota bene: This is the son of Marius and not the Marius
about whom we have been reading. His son replaced him as consul in 86 BC
after his father died only a few days into his term.
Marium iuniorem:
The iuniorem further emphasizes that
Sulla is fighting not his original opponent but instead Marius' son, "the younger Marius."
XV milibus [militibus] occisis:
Supply militibus. Translated literally as "with 15,000 [soldiers] having been killed"
AblAbs
CCCC [milites] de suis perdidit: The number (of soldiers) serves as a direct object, rather
than fitting with de suis.