How Rare would it be to acquire a Gargantuan Squiggoth? Would this be modified if the Rogue Trader in question possessed the Peer (Orks) Talent?

Peer (Orks)? What does a person have to do to get this talent?

As with all Greenskin organisms, the Magos Biologis of the Adeptus Mechanicus have identified Squiggoth's as fungal lifeforms, though they are fairly rare and can only be grown by the specialist breeder Pigdok of the Snakebites Clan. The size of the Squiggoth is determined by the effectiveness of the feed-formula the breeder gives his creature. Less-effective formulas may result in puny Squiggoths while high-quality formulas will see them keep growing and growing

Given that Gargantuan Squiggoths are themselves very rare and only the Snakebite tribe tends to have them at all I would put the acquisition value at Near Unique and would not even permit an acquisition check unless the one making the check was an Ork with the Peer (Snakebites) talent.

Pass a suitably difficult disguise check to dress up as an ork, kill the Warboss, and take the legendary hat that bears his mantle.

Pass a suitably difficult disguise check to dress up as an ork, kill the Warboss, and take the legendary hat that bears his mantle.

Given that Orks are a fractious, rowdy bunch who fight with each other as often as they fight with others shouldn't that at most give someone a Peer(clan/warband) rather than Peer(Orks)?

As I understand it there are a number of different species used as Squiggoth by the Orcs - if you can find out their homeworld or an orc conquered world they have spread to you could go hunt your own.

Alternatively you could substitute any suitably large behemoth class creature - training them is the tricky bit - might be easier if they are herbivores.

Gargantuan squiggoths are rare but there are ork war bands that have them. You should however only expect to see them on a world which has had an ork presence for a long time, the initial stages of an ork invasion tend to lack squiggoths. The spores must get time to spread and all that.

Peer (Orks) should definitively help and would be a good way to find an ork with a Gargantuan Squiggoth that wouldn't mind getting into the freebooter business.

You should however note that you won't get a tamed creature, you will get a barely controllable mountain of muscle and anger that hopefully doesn't try to kill your own men too much.

I would suggest making acquiring the squiggoth an endeavor in its own right.

Peer talent is usually used to modify the Commerce or Barter check, which then modifes your PF for that Acquisition check by +/-2 for each DoS.

A more effective way to go about it would be to purchase a baby squiggoth and then negotiate for a supply of a feed formula that should allow it to keep growing until it becomes a gargantuan squiggoth.

Of course you won't know if you've purchased the good stuff until the squiggoth either does or doesn't keep growing but on the plus side you've got a lot longer to try and domesticate it.

I wouldn't allow it as an Acquisition - more like an Endeavor when something is "Near Unique" and I dont want to allow it to be bought, traded for, or otherwise gained without risk (the verbiage in the books does say a GM can just disallow the possibility of acquiring any item as they see fit)...

I'm an advocate of Risk vs. Reward

Morbid