Two immediate caveats--we don't know that much about Maximus' biography, other than he does not seem to be a senator, and that he is from Spain, where he has a decent sized estate. Secondly, Gladiator, while a fun film, is not particularly worried about historical accuracy.

The term "general" is used loosely, but in the empire the most fundamental type of commander would be the legatus Augusti pro praetore, who served as provincial governors, technically as the personal representatives of the emperor, who governed the militarized provinces through their imperium. Only one militarized province (i.e. with legions) was governed by an equestrian prefect, Egypt, and this was because the province's grain supply was considered too sensitive to entrust to a senatorial governor. Most legati Augusti had held the office of consul, and so were not merely senators, but very high ranking ones at that. Legati legionum, the men in command of individual legions, were generally senators of praetorian rank.

Note that this will change over the coming century, as the so-called "Third Century Crisis" sees a series of military reforms that largely removes senators from the chain of command, and creates an equestrian officer corps largely of provincial extraction (especially from the Balkans). In many ways Maximus' thin biography as an equestrian (it seems) officer and provincial who has never been to Rome fits better for a "general" from AD 280 rather than AD 180.

It is possible that Maximus held a more subordinate equestrian position and that in the movie Marcus Aurelius simply decided to delegate a lot of tactical decisions to him for fighting shown onscreen. The battle we see is actually pretty small, seemingly involving hundreds of extras, so perhaps a vexillatio against a small tribe. Perhaps he was the praefectus castrorum, usually assigned to legionary camps but occasionally found commanded detachments; if we want to consider this a "general," then a brigadier at best.