It was "sort of" clarified in here https://forum.corvusbelli.com/threads/forming-fireteams-a-summary.3553/page-12#post-238388 I asked, do Wildcards Join or do they Replace. They "sort of" said Replace. They can't replace those that need the Haris or Duo skill though. That troop still needs to be there. The problem is the phrase "can be part of any Fireteam of this Sectorial Army." That wording can mean both Join or Replace. I was looking for concrete Yes and No answers but not getting them.
sadly no. This is IJW and other fellow forum users take on the rules - but not official. Wildcards has no official rule other than the sentence in the profile. You will not find an official rule that wildcards are allowed to "replace" troopers that are needed to fulfill the requirements for fireteam composition. Let's stop this fruitless discussion. Neither me nor anyone else has a valid solution for this lack of rules.
I assume it was just easier to write "may join any fireteam" than it was to write "Special Fireteam: Duo. 1 Rafiq Remote FTO or Authorized Bounty Hunter or Father Lucien Sforza, Registered Bounty Hunter FTO or Miranda Ashcroft, Registered Bounty Hunter or Valeryra Gromoz, Mercenary Hacker + 1 Maghariba Guard or Authorized Bounty Hunter or Father Lucien Sforza, Registered Bounty Hunter FTO or Miranda Ashcroft, Registered Bounty Hunter or Valeryra Gromoz, Mercenary Hacker."
Hows this: Wildcard troopers may replace any fireteam member except for those that must have the Haris or Duo skill, unless they have that skill themselves.