MongoDB - _id as String instead of ObjectId ? Relationship between collections or duplicate data?
Is it a good practice to store networks in a separated colletion?
(attention to the requirement above)

It depends on your app and how do you use them, but I can say collection design is okay.

Is there any compliance/convention issues building a relationship
between these two documents (like an SQL DBMS)?

Generally, No-SQL databases don't use for relationship collections.

And, I was wondering... can I store _id as String instead of
ObjectId? I've read some advices, but I don't have a conclusive
opinion about it. e.g : ... network: { _id: &quot;192.168.1.0/24&quot; }
(uniqueness guaranteed :D)

Yes, you can. BTW, uniqueness guaranteed by mongodb. Because _id field has a unique index by default.
db.inspections.insert({ &quot;_id&quot;: &quot;192.168.1.0/24&quot;, &quot;test&quot;: 1 })