Can a Hive Mutant take any career open to a Hive World character? While Scum seems to fit quite well (except for the Fellowship-strong leanings of many of the advances) and Guardsman - especially with the Penal Legion option - can work too, some of the others seem out of sorts. Can a Hive Mutant really become an Arbitrator (and potentially a Judge)? What about a Hive Mutant Cleric, Imperial Psyker, or Tech-Priest? For that matter, can a Hive Mutant eventually become an Interrogator or even an Inquisitor? And while a Hive Mutant Death Cult Assassin may be possible, could one become a Vindicare Assassin?

To be honest, which and what careers a hive mutant character should or could follow depends entirely on the style and direction of your campaign and the judgement of the game master. Just a few thoughts which came to my mind reading your post:

Scum - self explanatory I think, like you already pointed out.

Guardsman - fitting as well, especially if you use 'guardsman' as a more generic fighter / bounty hunter / soldier / whatever (tranch war anyone ?).

Cleric - this one can be used in a wonderful way using the background package 'heretical faith' (or was it 'cult', don't have my books with me atm.) from the RH (mutant priest of the Pale Thong, etc.)

Arbitrator - now in my games that'd heavily depend on two things: Did the character mutate before he was hired by the Arbites (I.E. mutated at birth, etc.) or during his employment, maybe (s)he was somehow contaminated while one duty in the underhive. A whole new venue of roleplaying opportunities opens up ... Of course it might very well be your mutant arbitrator is retired by now, and maybe even sentenced to a fiery death at the stake ... enter the radical inquisitor.

Assassin - speaks for itself I think.

Psykers - I'd prolly allow a hive mutant psyker as well in my games, it seems to be a fitting combination, yet after all it heavily depends on the background story the individual player comes up with, as usual.

As for techpriest, I'd honestly say no, yet of course it again depends on your mutual interpretation of the mutant background. There sure are a lot of things which yet might mutate a servant of the omnissiah ...

Inquisitors, Interrogators and everyone and everything in between ... after all it really really should depend on the grade, grossness and sort of mutation(s) your character is suffering from. There are quite a few mutations which could be easily concealed for extended periods of time, allowing a minor mutant to pass as someone more or less untainted for quite a while. It all depends on the circumstances, the individual player and character, and the story you're trying to tell.

Also note, there's a (general) transition package in Ascension which literally purifies your character, stripping him or her of all previously acquired mutations and malignancies at a price.

It all depends on the type of mutation (read: ease of hiding) and when it occurred in the character's life as to what careers are open to them. RAW, anything available to a Hiveworld is open to a Hiveworld mutant, but logic dictates (as with anything else) that a plausible story is constructed. IMHO< as with crafting any story, any rule or convention is mutable so long as it works within the aims of the story itself.

Take our arbitrator, for example. He's a mutant, built loosely in accordance with the Hiveworld background (I gave him a slight bonus in gear because he wanted to start as a mutant in accordance with the fluff he had created, complete with characteristic damages and all) since the RH wouldn't come out for another few weeks. His reason for being a mutant and somehow surviving in the Arbites?

The only thing "wrong" with him per se is his eyes look a little weird, and he has a hard time seeing in the light; he just wears photovisors everywhere. In fact, they're trademark items of his now. Even at the end of our most recent campaign when the acolytes were all awoken from their bio-restoration crypts, given a brief scrubdown and handed a neat medical gown before being ushered off to see the Lord Sector (he was very impatient to see the group who had saved his sector on the cusp of its destruction), he nabbed a quick pair of shades before making the visit.

So it can work. Just don't expect to be a sororitas with tentacles, or a wyrdling techpriest, for example. Unless you do a **** good job of the fluff.

The mutant in question is going to have Brute and Thick Hide. So he's bulky with overdeveloped muscles (not necessarily a sure sign of mutation) and has a heavily scarred skin (not too uncommon in the Imperium).

On careers:

Most careers (barring the AS) are somewhat open to interpretation as to what they actually entail. Just as a Guardsmen could be a PDF trooper or house guards and, as such, not an Imperial Guardsmen, so to can most any career be separated from its Imperial trappings. The Arbitor career can be used for bounty-hunters (and can take the Malfian Bloodsworn alt rank) which I'm sure would not only be beneath the Adaptus Arbites but might be some kind of Imperial crime for an Arbites to sell their services out in such a manner. So, for the mutant, it all really depends on their story and the spin you place on the career. A few ideas:

Adept: a Book-Slave, his spine bent from years of blessed data-hauling. He works in the deep dark depths of the census house storing and collecting records from 800-1,000 years ago for dedicated researchers who should not be made to endure the horrors and conditions of the Sunken Data-Vaults. Down in the dark, pulling and storing records and slaving for the knowledge and gain of others, he has come to learn things himself...

Assassin: The Apex Murderer. 700 years of selective breeding and alchemical rites preformed by the dark Order of His Golden Radiance has produced the perfect murderer to let blood in His Gilded Name. Unfortunately, the breeding and alchemical processes that have been conducted for nearly 35 generations have also left unfortunate scars and side-effects on their perfect (and some might argue heretical) murderer...

Arbitrator: The Bloodhound. An unfortunate twist who was used as a tracker and war-dog by the local precinct of Adaptus Arbites. Scheduled for destruction once his purpose had been served, papers got jumbled and he was forgotten long enough to seize on an opportunity and slip out. The Bloodhound is at large now, and he learned how the highest legal authority works and how to avoid it and most oter authorities as well while he plies the only trade he knows, the trade they taught him -running prey to ground. Now instead of serving the servants of the Emperor, he serves the rich gelders and noble houses who don't mind dealing with a twist if he gets the kinds of results that he is rumored to -just as long as the mark is tracked down and returned to them, in pieces or whole, and the gelt flows, everyone except the prey ends up on top...

Cleric: The Shriver. A mutant filled with righteous hatred for who and what he is, his body covered in lash marks and deep gouges from his own hands, he preaches to the down-trodden, the criminal, and worse trying to get them to repent, to seek absolution for their ways and join him in saving their souls by committing their bodies and lives to battling sin and corruption where ever it festers. Slowly a congregation of twists and dregs build around him and in their wake, bloody burnt atrocities smolder, the remnants of the twists and gangers who wouldn't repent and dedicate their lives to the punishing service of He on Terra and the eradication of any and all of His Enemies.

Gaurdsmen: A Twisted Traitor. A former freedom-fighter on Tranch, he as only ever known hardship and violence. He knows how to fight and kill and the ways of war, his sort life having been soaked in blood amidst the roar of artillery. He, however, is one of the few twists who turned on the Throng, and even rarer still, he yet continues to draw breath. While most of the traitorous twists that the Inquisitors turned were purged once their purpose had been fulfilled, the Inquisitor this brute answered to saw more use in him yet...

Imperial Psyker: The Wracked. It's bound to happen to any psyker who pulls on the warp too much, who forget the lessons and care that they had been taught, and it happened to this poor unfortunate husk that was once a man. He felt the tendrils of daemonic thought entering his head and before he could open himself up with his mercy-blade, he was lost -for a time that is. The thing that was once a psyker was captured by the Agents of the Dark Room, mysterious exorcists with strong hidden connections to the power elite. A holy and almost blasphemous battle with the daemon was waged by these exorcists eventually casting it back into the warp. While the psyker has been purged and cleansed of the daemon and has since sworn solemn oaths of fealty and secrecy to the Dark Room, he will bear the marks of the possession until the end of is days, a continual reminder of is carelessness...

Scum: ...Just a twist doing what twists are want to do...

Tech-Priest: A Twist-Tech. A former student of the Mindrusted Magus Manfred Magnus of Senophia-Magna (say that one five times fast, go on!) as well as the walking bio-alchemical experiment of that madman, he has finally broken free of the Mad Magus Magnus of Sinophia-Magna and found his way off of that accursed planet by way of a ship of ill-repute needing anouther engineseer of dubious morals and a lose interpretation of tech-heresy. As such a "priest", the Twisted-Tech has been making is way across the sector heiring his dubious services out to those who need a technical eye of a less discerning nature then the more upstanding Machine Cultists in the sector.

As for how any of these would ascend? While I don't yet have the book yet so i might be wrong, I'd have to say not very well. The Imperium isn't famous for being an equal opportunity employer after all and it doesn't even have a glass ceiling, just a big electrofied ceramasteel plate riveted into place. Such a mutant might be able to ascend (more then likely as a Desperado) but as other things as well depending on his story, how well he is able to hide his mutations, and how much play-room the ascended careers have. Off the top of my head and without ever having seen the book, I'd say that Judge, Inquisitor, Interrogator, and Vindicare are right out. The others might be doable all depending on the whos, whats and hows of the mutant and the careers.