As has been said earlier, PMCs might have access to old fighters, unless you're dealing with a monster corporation like the one in Derelict. Sync illustrated this well in a way, where the best you got were Hercs.
If pirates have military-grade fighters, they're probably trickle-downs from the PMCs, so if those fighters were last decade's news, then these are...
Otherwise, pirates/shady civilians will rely on civilian designs which are modified to suit their means. And now that I think about it, an Elysium transport would be a viable civilian/military design without too much debate on the matter. A civil variant of the Elysium would probably not need the Subach turret, but in wartime there exists the possibility that your vessel could be impressed into service... I don't know...
Thus, pirates/unlicenced PMCs/mercs would have access to very out-of-date fighters or derivatives of civilian craft; unlicenced manufacture of combat craft also is a possibility, for which matter I mention the PF Kulas once more.
The ML-16 is archaic. If it's that old, we might as well assume there were weapons before it that were much more primitive during the Great War. Most Great War campaigns use this as a useful storytelling/technological element as well. The only reason the Avenger would be more widespread in service would be that it's a more modern design - you'd assume many of the old weapons would just end up getting recycled. However, decent ammunition for the Avenger would require a specialty manufacturer - remember that Avenger rounds could pierce shields. Even sub-par ammo would be pricey as you're shooting a very big piece of metal into space, and a lot of it, too.
That said, the savvy pirate/"entrepreneur" might still use ballistic weapons, as they never go out of style, but they'll probably be civilian designed weapons or complete retrofits. However, the smarter weapon would be a laser/Subach sortof weapon. That too might be civilian designed. Missiles and ordnance would be very simple, I'd expect to see dumbfire rockets/bombs or heat-seeking weapons. Of course, it's really up to the writer to make a good story out of what he deems reasonable to assume.
In this case, what about post-NTF fugatives? Those are aways fun...