Hmm... finally got time to really dive into this one....it's even more complex than I thought
hangers on and brutes, second paragraph:
"Hangers-on are primarily used in Campaign play, although some will prove particularly useful in Skirmish play. Brutes are purchased specifically to add some extra muscle and prowess on the battlefield and can be fielded alongside the rest of the gang normally. Either at gang creation or in the Update Roster step of the post-battle sequence, players can recruit Hangers- on and Brutes. These are purchased with credits from the gang's Stash in the same way as new fighters. A Fighter card is filled out for each and they are added to the gang roster. Most Hangers-on and Brutes have various options for their equipment – these must be decided when they are recruited and cannot be changed later."
Brutes, first paragraph:
"Unlike other Hangers-on, Brutes are purchased with the express intention that they be fielded in battle and are treated like any other fighter when selecting a crew. Unlike other Hangers-on, Brutes will gain Experience and Advancements in the same manner as a Specialist. Unlike Hangers-on, Brutes will not leave the gang should they suffer a Lasting Injury that makes a change to their Fighter card."
And then from the warp horror:
"A Warp Horror may take any of the following upgrades, in addition to its horrific appendages. Note, during a campaign, a Warp Horror can also do this in the post-battle sequence to represent it mutating: " <it then lists the upgrade options>
From the first bullet point of New Equipment
"Any fighter or Crew may discard any Wargear (including a vehicle) they are equipped with in favour of new Wargear. Any Wargear discarded in this way is placed in the gang's Stash and may be given to other fighters. Note that this means Wargear can only be discarded if being replaced by an alternative item that fulfils a similar purpose."
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My read is that I'm limited to 3 weapons. I can acquire more than 3 over the campaign, but over 3 would require excess to be traded for an existing one, which is then sent to the stash.
Further, being able to change weapons at all, is apparently unique to the Warp Horror as far as brutes are concerned.
That said, I'm still somewhat unclear, since it describes them as "upgrades" and I'm not clear if they should be allowed to be sending their warp horror mutation weapons to the gang's stash.