As you know, is it legal for Google rules to have multiple Google Play Developer Accounts?
Thank you.
Yes, it is legal, even if you are an individual (as in “not a company”).
But they reserve the right to limit at any time the number of accounts you are allowed to have.
It’s very clear in the Developer Terms you have to agree to create your account - didnt you read it? 
https://play.google.com/intl/ALL_it/about/developer-distribution-agreement.html
- Credenziali dello Sviluppatore
8.1 Lei accetta di assumersi la responsabilità di mantenere la riservatezza delle credenziali di qualsiasi sviluppatore che possono esserle fornite da Google o che può scegliere lei stesso, nonché di essere l’unico responsabile di tutte le applicazioni sviluppate in relazione alle credenziali dello sviluppatore. Google può limitare il numero di Account sviluppatore fornito a lei o all’organizzazione o azienda per cui lavora.
LOL
Do you understand what they mean by organization and developer?
I mean, as I undertand and understood before, I can have a company called “Beppi’s” and more developers that have their own personal GP accounts, but publish under the “Beppi’s” company.
But my question is the reverse: I am an individual developer, can I have multiple companies? Can I publish one app with “Beppi’s” and one app with “Rottz’s”?
I don’t understand…
From what I understand, yes, you can have as many accounts as you want as an individual developer.
Quoting the English version:
Google may limit the number of Developer Accounts issued to you or to the company or organization you work for.
It clearly states “to you”, so it allows an individual developer to have multiple accounts (but they reserve the right to limit that number).
You can have as many as account you like, and you can also publish different apps for each accounts. (They should be different apps). However, if one account gets banned for a reason, you can also lose all other accounts.
Thank you.
I suppose that there is no problem to keep one single account on AdMob that refers to two Google Play Accounts, right?
Of course, who would use same google play account (for publishing apps - same email) for large money transactions, admob account (email) should be secret and kept safe!