Also, in a functioning ship market I would expect many ships to be broadly similar, especially common cheap merchant hulls, so the asymmetry often has to come from position, surprise, readiness, or intent rather than wildly different ship stats. And likewise, desperate pirates are probably not showing up in Imperial super-fighters, right? Rather cheap, modified, stolen, obsolete, or poorly maintained ships?
With respect to this point, whilst merchants are broadly similar it doesn't take much modification for them to become significantly different.
For example, the default Class A trader is unarmed. It has two hardpoints for weapon systems but they are by default not populated. It is an expected modification for something to be fitted, but the choice could lead to wild asymmetry. If one ship has fitted a pulse laser and the other has fitted a beam laser then the beam laser ship can be hit without being able to hit back. If one ship has missiles then it may be able to hit the other ship before the target is even aware of the missile ship. With a Class A a single missile hit could be terminal as they will tend to cause a cumulative damage critical.
Pirates may be using captured merchantmen and they may not be able to upgrade the drives or armour easily (since it requires Class A or B port facilities) so keeping them stock is entirely plausible and to be expected. They should not be running with damage unless it was inflicted recently as damage can be repaired by the crew as long as they have time and spares (or if they have access to almost any starport). If a component was destroyed it cannot be repaired, but that ship likely isn't viable as a platform and the encounter won't happen.
You could randomise the weaponry of the pirate with a table and that will dictate the tactics they are using.
If they can hit with a missile from distance then they will probably announce that while the player ship is approaching (since it is much harder to turn around and flee) and call for surrender. The players then have the choice of complying or trying to run away (and weather the storm of fire in the interim). If the pirate only has a single launcher in the turret a good crew with weapons that can be turned to point defence might well be able to manage. If the pirate sees that it is just wasting missiles it will probably call off the attack since each shot represents a cost.
If they can hit with a pulse laser at distance then they will probably do the same as with a missile. However due to the shorter range and each shot being free, they might pursue more vigorously. Even if the player ship has a sand caster it won't necessarily be effective (especially in vector combat) and it has limited ammunition, all it can do is buy time. A pulse laser is probably the ideal weapon for this sort of situation and it has a good chance of punching through even respectable armour levels.
If they only have beam lasers then they will need to be quite close and will probably go for an ambush once refuelling has started. The sand caster here is likely to be more effective even in vector combat since a stern chase is likely. If you decide that the atmosphere attenuates the damage of a laser a sand caster might not even be necessary. If you are lucky the armour on even a Class A can entirely block a beam laser. This type of combat is likely to be the most fun to fight out as the player ship needs to take quite a few hits before it is going to be significantly impacted.
If they are entirely unarmed they may just sneak up and attempt to board (either by linking) or space walk (though in atmosphere this would be exciting to say the least). This is unlikely as a primary threat vector, but if at least one of the small craft is armed, then it might be the follow-up once you had surrendered. Once craft would approach to land boarders while the others stand-off to quickly punish any attempt to resist.
A low threat pirate encounter could be several (3+) small craft equipped with beam lasers on their firm point. Each will need to be detected, tracked and targeted separately which means player choice. A single hit on one will eliminate it allowing the players to quickly alter the odds (and give the referee a reason for the pirates to call off the attack). You are building in credibly balanced asymmetry as pirates are trading speed and dispersed attacks for individually weaker vessels. It is also credible that even hard up pirates are easily able to scrape up a few ships boat's or launches as they are cheap and ubiquitous. You can also have them piloted by mediocre crews who are also ubiquitous - every individual that fails to survive their first term in the Navy gets Gunner-0 Pilot-0. This is a credible source of disaffected, young tearaways looking to show "da man" they had the right stuff. A few ex-navy who washed out in their second term might even have come with a small-craft as a benefit. If a war has recently happened (or is in progress) you might have quite a lot of retired Navy types with no work (and possibly no home world). Drifter(Scavengers) or Scouts get Pilot-0 and Astrogation-0 and so are also particularly suited to operating close to Gas Giants.
If the small craft originate from an abandoned space station with a shipyard somewhere in system then you also have a later campaign option to smash the pirate base. If you want variety you can plunder the Small Craft Catalogue for ideas, but half the fun of being a referee is designing stuff and randomly cobbling together small craft can be an amusing exercise in itself. Vessels could be converted launches or dirt cheap asteroid weapons platforms with R-Drives.
If you place this base on a route the players need to use regularly then it will justify the amount of work you are putting into random gas giant encounters. The players have a chance to get the lie of the land and negotiate how they are going to minimise the risk. There may be an established "toll" - the pirates may claim ownership of the gas giant. They may attack randomly as each hunting pack is commanded by a pack leader who have wildly varying motivations. Some are just trying to survive and would prefer not to take life, some might be more keen to prove their tactical brilliance and will value a worthy opponent, some might be mentally unbalanced by their war experience and may be still fighting that war. Resourceful players may conduct some investigation to attempt to reduce the chance of a terminal encounter.
As a player I would much prefer there to be a reason for even "random" encounters.
