Hi!

For your first question:

Can I do this with maybe 2 droplets, that I have at the end 2 different nameservers?

The short answer is “Yes” You can setup Virtualmin on two droplets and have one be the primary (master) and the other be the secondary (slave) for DNS.

For your second set of questions:

But how can I do this with my new Webserver? Can I (in this case) do the same with my new server? Or do they “bite” each other?

If you have already created a new web server without Virtualmin, there is a very good chance that trying to add Virtualmin to the new web server will break any configuration that you have done.

Here’s an older answer that covers the bases:
https://www.digitalocean.com/community/questions/can-virtualmin-webmin-be-installed-on-a-droplet-which-already-has-few-active-wordpress-websites

Here’s a DO tutorial on setting up virtualmin on a droplet - feel free to leave out the parts you do not need.

https://www.digitalocean.com/community/tutorials/how-to-install-virtualmin-with-webmin-lamp-bind-and-postfix-on-ubuntu-16-04

I hope this helps, if you have more details about your proposed setup please feel free to share them so we can better advise. Let us know if you have any more questions about the above and good luck!