Obviously, option A (MY LORE SHALL REIGN SUPREME!), and human, because humans are mediocre at everything, and this is a well-defined fact. Not to mention it's better to be human, because then the phrase "Ammonia-filled atmosphere" does not immediately require a check to see if we are some sort of twisted ammonia-breathing race of armored sauroids.
As far as government...Democratic. I'd say modeled on US gov't, because then we can have competing politics among players. The Senate is much smaller (Say...10 members. Filled by NPCs if there aren't enough players) and a President. If necessary, (Read: If GM wants to go THIS DEEP into the politics of it all) we can have a player-run Supreme Court with 3 or 5 members to pass judgement on players and perhaps even some NPCs who run afoul of the government. All positions elected by people (Including SC, otherwise unfairness, since we won't ever die) every few turns. All names temporary, meaning cooler names for the Presidency, Supreme Court, and Senate are welcome. Relatively large government because we're on a spaceship and the government needs to regulate things. Perhaps we have some archaic rules ensuring that priority positions such as propulsion engineers, life-support techs, farmers, and biosphere monitors are all filled at all times, though that is not an issue now.
There is only one military branch, the Fleet, but it does contain a Marine Corps, for those times when shooting them from your ship just isn't working, and you need to shoot them on their ship. Since we're mostly a keep-to-ourselves sort of nation, we've never needed to conquer anybody, so ground warfare tech is lagging a bit, though there are some few APCs/IFVs on the worldships in case an enemy breaches the hab-domes and needs to be shot dead.
Other institutions: Free-business model, basically. Lots of cooperation (Comes from living on in a confined space from lots of other people), perhaps, but other than that, whatever you wish to do as long as you're not injuring, in any way, any member of the nation. So yes, perhaps we have a few mercenary corporations tucked in the back, a sneaky Assassin's Guild somewhere in the depths of the damaged quadrant of Worldship Three (Asteroid collision. Have not yet acquired parts to repair it), and a drug-running cartel basing itself out of Worldship Four's decrepit starboard aft hangar array. However, the mercenaries don't find employment on the ship, the assassin's don't kill any of our friends, family, or allies, and the drugs stay off the Worldships, so we don't really care. There's a megalomaniacal corporation owner who likes to kill past business associates who betray him, but they've broken our laws, so good for him. We have a government-sanctioned ship-raiding-and-stealing division for those times when an unpleasant group of people has a nice ship and we want it, and an unofficially government sanctioned black-ops piracy division for those times when it's someone who's broken our laws owns a ship and we can't prove he did anything.
As for robots, how about no? Everything is human-run, and there is only a few bits of automation. Perhaps the people have had some terrible automation-based catastrophe in the past that had made them distrust automata, or perhaps their parents just grew up walking uphill through the vacuum to school, both ways, and you'll do the work that needs to be done and not let some useless robot do it for you.