Questions About Fire Team Sizes

I am a new Kill Team player, I have a few painted Squads but I am curious about the wording in the rules, I am very new to table top games in general so I may just not understand the nuances of the wording or just how they are played.

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But the main team I have been using is Death Guard, using my Plague Marine Fire Team, I have been playing with someone at my FLGS who knows the rules and has been teaching me, when I play my Death Guard my list currently is

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Plague Marine Champion with Plasma Pistol and Power Fist

Plague Marine Icon Bearer

Plague Marine gunner with Plasma Gun

Plague Marine Fighter with Bubotic Axe, Mace of Contagion and Bolt Pistol

2 Plague Marine Warriors

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Now I was reading the rules and I have some Custodian Guard I wanted to play so I was reading their section of the Rules it was says that it has 2 operatives. Which made sense to me based on how I understood rules (which may have been very little) from the last edition, but I figured 2 Custodian Guard wasn't much but they are Elite so I get it.

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But reading my Death Guard section is says the same thing, that I can have 2 Plague Marine Operatives, and I am unsure what that means. Is my list not good because I have more than 2 "specialists" or am I only allowed to have 2 Plague Marines? I'd hate to have been playing with a list that the rules don't allow as I am trying to learn this game and I want to be fair to anyone I may play against.

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ALSO, how does it work mixing Fire Teams? I have some Rubric Marines and my brother has some Tzaangors, he is coming to visit me soon so I am trying to learn the rules so we can both play and learn together but all he has are Tzaangors. I am thinking he will want to mix them with my Rubric Marines after we play a few games so how does that work? I used to know the old edition worked on a points system, but I never played so I can't speak to how effective it was, it was just easier for me to understand when I was building lists in Battlescribe.

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If anyone can help me out with this, I would greatly appreciate it!