FYI just to clarify some of the things you've said:

>All that stuff can stick to the walls of the ventilation system, creating biofilms where amoeba and other microbes can feed on

Biofilms are formed by bacteria, not the other way around. They do contribute to persistence and survival as many of the individual cells within a biofilm are not actively metabolizing, making them need fewer nutrients and making them very hard to kill

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>Legionella lives in some of these microbes

Legionella is a microbe (gram-negative bacterium), and it can live within other organisms (like amoebae) as an intracellular parasite, but it does not necessarily have to

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>Legionela needs Iron to survive, so they can easilly get it from the corrotion ocurring from the "weathering" of the ventilation system

Almost all bacteria need small amounts of iron (and other metals) to survive, but they don't need to be on a surface made of iron to acquire it. They can grow on surfaces made of many different types of materials