Oh no, absolutely not. First and foremost, as a Roman citizen, one of your rights was that you could not be sold into slavery - well, at least not within Roman territory (The Romans that Hannibal or the Parthians or the Germans captured were used as slaves). Gladiator is a wonderful movie, but when it comes to actual history, it essentially gets everything completely wrong. Let's start off from the start, just concerning Maximus here. First off, his first name would have been Decimus, not Maximus, but that's a nitpick.

So, a Roman general. In Gladiator, we have Maximus shown in almost the light of Cinncinnatus - he's a landholder with a small household and a farm and a happy, loving family in the country, no slaves in sight, etc etc. That would never have been the case. Generals in the time of Marcus Aurelius, were drawn from the upper classes, of which small landowners were certainly not. A man of Maximus' status would have been one of the top men in Rome (Remember, Aurelius supposedly wanted him to be Emperor!), right in the middle of the politics, with a rather large house and a whole passel of slaves who would have been considered an integral part of the household. Long story short - he would have been an extremely important man.

So, what we have is an extremely important man - at the very least a Senator at this point - being assaulted, his property (ignoring the fact that he would DEFINITELY have had much more than just that) burned down, and being sold into slavery - and accepting that fate. It would never have happened.

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As to the rest of your comment, while there's a possibility that a foreigner walking the streets of Rome at night may have been mugged and sold to the slavers, it's highly unlikely. Slavery was a completely legitimate business and a huge part of the Roman economic system. Slave traders were a type of merchant. If you get the reputation for stealing people off of the street, it's not a great reputation, especially when you had other people who could vouch for you. For a quick example - it's a couple of centuries off, but it should be helpful - we can look at the slightly legendary tale of the decemvirs of the Old Republic in the third book of Livy. I'll give you a quick TL;DR of the intro to this situation, then move on to the actual story that addresses your question.

The Roman people were irritated with the lack of formal, concrete laws that they had, and the plebeians were rather riotous over the extreme power that the patrician class was taking into their hands via the consulship. So, as he notes...

It was decided that a body of Ten (hence called the "Decemvirs") should be created, from whom there should be no appeal, and that all other magistrates should be suspended for the year. There was a long controversy as to whether plebeians should be admitted; at last they gave way to the patricians on condition that the Icilian Law concerning the Aventine and the other sacred laws should not be repealed.

So the Decemvirs were elected and during the first year of their office, they wrote down the first ten "tables" of Roman law. The next year, the people loved the system so much, they elected new decemvirs - but one of the old ones (Named Appius Claudius) wanted power. So he rigged the elections, had himself re-elected, and made sure the nine others who were elected were his puppets. The second Decemvirate crafted two more "tables," but, according to Livy, everyone was despondent due to the tyrannical nature of this new government. It all came to a head with the (attempted) rape of Lucretia (starting at 3.44 on the aforementioned link).

A Roman girl, utterly virtuous (remember, this is legendary - take it with a grain of salt) in every way and raised by a good, strong, upstanding Roman family, caught the eye of Ap. Claudius. She was a plebeian (In this time period, that's the non-nobility type class), and was already betrothed to another fine, young, upstanding, practically perfect, people-loving Roman. Here's what happened next:

This girl, now in the bloom of her youth and beauty, excited Appius' passions, and he tried to prevail on her by presents and promises. When he found that her virtue was proof against all temptation, he had recourse to unscrupulous and brutal violence. He commissioned a client, M. Claudius, to claim the girl as his slave, and to bar any claim on the part of her friends to retain possession of her till the case was tried, as he thought that the father's absence afforded a good opportunity for this illegal action. As the girl was going to her school in the Forum - the grammar schools were held in booths there - the decemvir's pander laid his hand upon her, declaring that she was the daughter of a slave of his, and a slave herself. He then ordered her to follow him, and threatened, if she hesitated, to carry her off by force. While the girl was stupefied with terror, her maid's shrieks, invoking "the protection of the Quirites," drew a crowd together.

Needless to say, this didn't go over well - she was kept in custody until either her father or her betrothed were able to speak in her defense. Her betrothed was able to delay long enough for the father to return, but unfortunately, even that wasn't enough - Appius Claudius was driven insane with power and refused to accept that she was a Roman. I'll finish off with Livy's description, because it's just so damn GOOD. The man was a wizard with the pen.

The decemvir, utterly abandoned to his passion, addressed the crowd and told them that he had ascertained not only through the insolent abuse of Icilius on the previous day and the violent behaviour of Verginius, which the Roman people could testify to, but mainly from certain definite information received, that all through the night meetings had been held in the City to organise a seditious movement. Forewarned of the likelihood of disturbance, he had come down into the Forum with an armed escort, not to injure peaceable citizens, but to uphold the authority of the government by putting down the disturbers of public tranquillity. "It will therefore," he proceeded, "be better for you to keep quiet. Go, lictor, remove the crowd and clear a way for the master to take possession of his slave."

When, in a transport of rage, he had thundered out these words, the people fell back and left the deserted girl a prey to injustice. Verginius, seeing no prospect of help anywhere, turned to the tribunal. "Pardon me, Appius, I pray you, if I have spoken disrespectfully to you, pardon a father's grief. Allow me to question the nurse here, in the maiden's presence, as to what are the real facts of the case, that if I have been falsely called her father, I may leave her with the greater resignation." Permission being granted, he took the girl and her nurse aside to the booths near the temple of Venus Cloacina, now known as the "New Booths," and there, snatching up a butcher's knife, he plunged it into her breast, saying, "In this the only way in which I can, I vindicate, my child, thy freedom." Then, looking towards the tribunal, "By this blood, Appius, I devote thy head to the infernal gods."

Hopefully that gives you an idea of the Roman ideals towards being enslaved. Very much "death before dishonour" in that aspect, and the number of times that Livy refers to "the hideous crime" and the caricature of Ap. Claudius as the perfect villain...a Roman would not be enslaved in Roman territory. Hope that answers your question :)

No, not quite. There was a clientes-patrones system in ancient Rome, where pretty much everyone had a patron (someone who was richer and/or more powerful than you). That way I suppose you could potentially be taken off the street in the way you suggest and made a slave, but your patron should get you out of that situation. That is what is expected of him in return for your loyalty to him as a client.

Taking the case of Maximus, his patron is actually Marcus Aurelius. When Marcus Aurelius dies that patronship is passed to Commodus who then has become the dominus (head of the household). A big part of the movie plot is Commodus hatred for Maximus (for obvious reasons), which makes him dismiss his patron responsibilities towards Maximus and essentially renders Maximus utterly exposed.