@gingerroebro
It was in the old rules. In the new ones, the worst result is that yous psyker explodes in a war power explosion blast that wrecks everything around him. In the old ones, he could get posessed, which gave him a bonus of +2 to all his stats (a big buff) and inmune to psycology, meaning that a posessed psyker could engage 3 gangers and you could expect the psyker to win and want more. Also, since it's posessed he would engage any nearby unit, be it yours or the enemy. Dunno if they will implement this or not, because it also means that your psyker is dead because he is posessed by a chaos daemon, and your gang puts him out of his misery if he still lives at the end of the fight.
In all due honesty, i hope we have permanent gangs in the game because that would mean you would fight harder or easier fights from time to time. Some gangs with high reputation would have these special units while others would have simply more gangers instead of mirror matches like in Mordheim (enemy warbands always had the same rating as your, meaning that you always fought a mirror of your warband in terms of strength). Still, there is always a way of bring down of these huge guys. But you have to remember to pray to RNGsus for it to work properly.
I know we want our favourites, but in this case i have 2 reasons to think that a chaos space marine in the underhive of necromunda is an almost impossible scenario: chaos space marines attract a lot of attention from everyone because of their huge rampages typical of blood crazy maniacs and the fact that there is a space marine base in Necromunda (Imperial fists if i'm not mistaken). That summed up with the stern control that Lord Helmwar keeps in the hive makes it incredibly unlikely that some Chaos Space marines are wandering the underhive. And so for Chaos Daemons, since they need a steady supply of chaos energy to stay in the material world (something that was possible in Mordheim due to the inmense amount of warpstone in the city). But still, finding some Chaos cultists in the underhive is a very high possibility, and the same goes for Genestealer cultists.
For the reasons of other races to be in the underhive, the Dark Eldar is the simplest: they need slaves. They will be glad to take slaves since Dark Eldar are known for stealing entire planets. And knowing that there is an Eldar Embassy and that the noble houses of Necromunda are entagled in illegal drug dealing, it's not impossible that they have some Dark Eldar pirates in their payroll (even tho it's more likely the opposite). For the orks, they always want to fight. If you tell them there are some really tough ass people in that shiny place in the distance, they will go there in droves in order to bash their heads in and take their stuff. And then, look for even tougher people to shank and take even more stuff. It wouldn't surprise me that some Orks from the Skull venture into Hive Primus to fight gangers from time to time just for fun.
When it comes to Chaos, they will usually want to bring down the entire planet, make everyone go insane and kill as many people as possible. All for the promise of power beyond this world and inmortality. And when it comes to craftworld Eldar, i doubt they would even consider meddling with humans, mainly because they think of themselves to be far avobe us filthy and lowly monkeys. The only times craftworld Eldar do something is to prevent the rising of a tomb world, to prevent future harm to their interests (such as colonization of maiden worlds and active campaigns against them) or in fighting the forces of chaos (their main enemy).