No. When designing trophy sets for PS3 games, one of the requirements is that all of the base trophies must be earnable without DLC. Sony will not let developers publish games that do not meet this (and many more) requirements.
I finally figured out what they were talking about. "The green ones" in that snippet refers to the rebels that appear in front of the building (not actual mobsters), and there's actually four of them. There are two rebels that respawn right there outside the building. They're the two standing in front of the sandbags side by side (there's usually a cop there too patting someone down up against a pillar, so it's a little tricky). The other two respawn down the street at the other side, again in front of the sandbags (there's also usually a second cop patting down a civilian there too). Apparently, these rebels also count as "mobsters" at least for the trophy's sake. I've already tested this out on three execution styles, and it counted them. So I'm happy I don't have to start a new game and play through it all again...
I've searched around Cuba a little to see if there were any other rebels anywhere and couldn't find any, so this appears to be the only spot they exist after you beat the game.
How do you tell a rebel from a cop?
It's confusing at first, because they both where virtually the same uniform. The rebels have a slightly lighter green uniform (at least I think they look lighter). However, the easiest way to tell if it's a mobster or not is to simply go stand in front of them. If it's a cop, they won't budge; they'll just stand there and stare at you (unless you do something to cause the police to chase after you). However, if it's a rebel, they'll start talking to you like "You don't belong here" and other lines and they'll try to attack you. They always start with their fists and they only upgrade to guns if you draw a weapon on them first.
The only hard part about doing it this way is the cops. There's always one there (at least there's always been one there for me). So when you start shooting and attacking the rebels, you're going to get an instant red circle (the cops want you). You'll end up spending more time trying to escape the cops. I've just been going over to the area beside the casino and strangling a civilian. This pops up a yellow circle so the police come, and usually after it goes away, one of them will be available to bribe. Once you've bribed them, then you can attack the rebels freely and shoot any cops that attack you without having to worry about more coming (cops always shoot at you if you do sometihng in front of them, but if you have the bribe still running, more cops won't come and you won't be wanted).
Best Answer
It's part of a map/character multiplayer DLC which is not yet released. If you group your ACIII content by category, it will show these trophies their own section from the non-DLC trophies.